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Related FAQs: Freshwater Acclimation, Freshwater Livestock Selection, Livestocking Freshwater

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/The Conscientious Aquarist

Freshwater Livestock Acclimating


By Bob Fenner


Of the common sources of livestock mortality, a great many organisms are lost simply to inadequate or improper acclimatizing.  Indeed, collectors, distributors, transhippers, wholesalers, retailers and hobbyists suffer most of their losses in the process of receiving new livestock.

The moving and placing of freshwater fishes, invertebrates, and live plants into a new environment there arises the possibility of physical, chemical, biological, even social shock. This article details how to avoid these stresses altogether, or ameliorate the effects of stress after it has occurred.

There are a few general types of acclimation techniques; depending on the stock's apparent health and conditions at shipping; all are to some degree appropriate and successful. Many losses can be avoided with the information provided here. Whichever method you employ, do so systematically.

Understanding the Need for Acclimation: An Overview of Livestock Stressors:


    Water is the standard for "specific heat"... it holds and gives off more energy per mass than any other substance known in the universe. Hence, large volumes change temperature very slowly. This is not the case in shipping bags or small aquariums. Too many organisms, even a small change in temperature over a short time is detrimental; thus attempts are made to equilibrate the temperatures in new and old environs. Often this is accomplished by floating the plastic shipping bags in which the livestock is transported in the aquarium. This process should continue for 10-20 or more minutes, depending on the amount of water shipped, the temperature difference, and the apparent strain the stock is undergoing. It's often a good idea to monitor the temperature in both environments.

Opening the floating bag is not a good idea as the concentration gradient of oxygen may be higher if left sealed (In pure oxygenated bags) and an open bag usually reduces surface area and therefore gas exchange. Keep this in mind and maintain some surface area in the bag and/or add a mechanical aerator (an airstone) while adjusting for temperature if you open the bags.

Remember, there are profound differences between mammals such as dogs, cats, ourselves and other warm-blooded animals; aquatic life is "cold-blooded". Be aware that thermally-shocked fishes may appear and behave "normally" on arrival and placement; only to die mysteriously within a few days. The biochemical and physical damage resulting from thermal shock often catches up later.

Exception Note: if there is a time constraint or if new specimens under severe duress, they are often better off being introduced immediately. This is much more preferable if the water they are being moved into is warmer than the transported water. In general, cold water shock causes a great deal more harm than warm water shock.

Finally, regarding an "old-wife's tale" about the detrimental effects of floating polyethylene bags. In the latter 1960's there circulated a myth that this practice (THE AQUARIUM, magazine, June 1968, Vol. 1 no. 8 series II) would kill fish. This is hogwash.


    It is also important that the aquarium lights be dimmed or turned off during this time; especially bright sources like halides, as they will heat the water in the bags too much, too soon. This results in decreasing gas solubility in the bag, increasing metabolic rate, frightening the livestock, and generally cooking them. Many tropical organisms are intolerant of sudden or wide temperature range or outright photo-shock.


Freshwater can vary considerably, chemically and physically from source to your system, even if originally from the same tapwater source. Often times the successful introduction of new livestock into a tank hinges on the relative chemical composition of the waters involved.

It can be a very good to disastrous idea to try reducing some of the chemical shock by mixing water from your system with shipping water after floating. Allow me to explain. Several "things" happen in an enclosed shipping bag as time ticks by... Even if "pure" oxygen (99+%) versus ambient (@21% by volume) is employed, the alkaline reserve of the water itself gets nicked down bit by bit, and pH will start to drop... carbon dioxide increases in concentration as carbonic acid further lowering pH... and very importantly, ammonia from the livestock in the form excreted wastes, secretion from the gills and body increases... This is enough of the picture we want to portray. Depending on how much of these changes in the water's make-up have occurred (the time in the bag, relative volume, mass of livestock...), you want to mix-waters or not. How to tell which if this is a good idea or not? For short trips from the dealer, let's say an hour or less, you're probably better off blending in the system's water to acclimate new stock. If you prefer, using a test kit for pH and ammonia will give you a window into whether to mix or not (if there's little appreciable ammonia or difference in pH). With any detectable ammonia and any measurable difference in pH, do not mix waters. A rapid elevation in pH coupled with ammonia is extremely toxic to almost all aquatic life. Of course, just like grading school papers, "when in doubt, count it out", and don't mix. To repeat: If you have a concern about whether it is better to blend a specimens shipping water with the system to acclimate it, don't. For long-range trips, and folks dealing with many or expensive animals, please read the article below on Guerrilla Acclimating Techniques carefully.

Do not put the shipping or mixed shipping water into your established system! The risk of introduction of chemical pollutants and undesirable organisms is not worth it. However, there are two scenarios in which adding shipping water may be beneficial: 1) When the receiving set-up has not been properly conditioned and 2) When "wild" stock chemical conditions are so different that even biologically polluted shipping water is better mixed in than all new "clean".

There are numerous techniques ranging from ingenious drip systems to simply adding a portion of new water to the shipping bag a few times every few minutes, to floating traps to....(see Jim Mortensen's June 1987 FAMA "If I Had Only Known"). The most appropriate and successful method I've seen is detailed below.

One Method of Acclimation:

After and while adjusting for temperature differences by floating then possibly slowly adding water from the system to the shipping water, net out the organism or pour it and the mixed water through a soft net. If at all practical, most new livestock should be isolated in a quarantine set-up, separate from your main display. This "way station" allows for a few important processes. One, to allow the new stock to "rest up", becomes familiar with captive conditions... w/o harassment from your established livestock. Two, to allow you a period of time to observe, assess the newcomers for health issues, and... Thirdly, to detect and possibly treat them for disease or pest removal.


Admittedly, acclimation is stressful of and by itself, and if your new livestock is in good shape and the new environment not too unsuitable, they could often be placed into their new homes with little ill effect. With attention to proper acclimatization, however you will minimize your losses, avoiding social and transit shock and MANY trials in treatment of pathogenic disease.

    Remember, it is important to prepare your holding system ahead of time. Generally it's also best to hold off feeding for 24 hours after the arrival of your new livestock. Lastly, always condition and quarantine your livestock before moving them again. Pay attention to these details and you will be rewarded with happy, healthy new additions to your system.  

/The Conscientious Aquarium Dealer: 

 Freshwater Commercial Acclimation Methods

This is more a treatise, instruction set for serious breeders, importers, wholesalers... but the basic premises and techniques bear exposing to home hobbyists.

By Bob Fenner


What's your biggest source of enjoyment from keeping aquatics? Probably the livestock with the emphasis on live. As conscientious consumers of the living aquatic environment we are faced with moral/ethical questions of "rightness" in what we choose to do in our hobby. How much wear and tear on habitats, loss of vitality, life is acceptable and for what acceptable reasons? Let's think about this; we have discerned effects on the planet, we should be aware of what we're up to.

An inescapable fact of life is its apparent antithesis: death. As difficult or unprofitable as its discussion may be the loss of aquatic life is equally inevitable. The magnitude of these losses at every level, collector, breeder, intermediary to end-user is very large; some reasonable estimates exceed ninety-plus percent in some cases... Some of these losses are unavoidable practically or economically. It is a fact that unlike amniotic mammals, the lower-vertebrates (fishes) undergo a great deal of development in later, free-living stages, with concomitant losses... and that pet-fishing in the business sense is an economic endeavor, done for profit. If cost were not a chief concern the trade might be very different indeed.

Where do most losses occur? At what point in the use of aquatic organisms is mortality greatest? You know where, and it's the same for all levels in the chain of production and distribution; at the transition of moving/acclimating/and stabilizing "new" livestock from one system to another.

Whence Acclimation? Definition of Opportunity:

A search through the cited scientific and my semi-exhaustive pet-fish hobby and business pulp, bound and ephemeral materials turned up surprisingly little of real use on the important topic of acclimation. Why? Is acclimation of such small consequence? Decidedly not. I put it to you again, ask yourself, other aquarists, the dealers when and where do they lose most stock? Are acclimation methods "secret", "hocus-pocus" maneuvers that convey special status or abilities on wizards of wet-pets? No, it seems to me that most folks are just too embarrassed to admit their losses; or their lack of "coming to grips with the realization of their own limitations".

It is my desire here to:

1) Acknowledge the dimensions of mortality that occur consequent to poor, inadequate or inappropriate acclimation.

2) State that these large losses occur across the spectrum of collectors, breeders, consolidators/transhippers, wholesalers, retailers, hobbyists.

3) That much of this loss is avoidable and

4) Offer my "Standard Operating Procedures" for marine and freshwater acclimation.

This discussion is intended to provide stimulus, some guidance and insight for others to adapt/adopt, and a jump on/off point for the bridge between the scientific and hobbyist literature. We possess a great deal of empirical knowledge in the hobbyist realm; the scientific world is prophetically swarming with disparate, fragmented facts and for-the-most-part unusable high-tech (immunofluorescent, bio-chemical, micro-,) factual material. Pet-fish magazine articles are followed and indexed by citation services. Maybe the twain shall meet sooner.

Individual's with a small collection, adding one or two specimens from time to time should benefit from the following guidelines for large-scale mass processing and are referred to Steve Landino and I's piece in 1989's FAMA.

Acclimation in General: Preliminaries

If it isn't obvious, these opinions are just that, the syntheses of my experiences in this field through the last forty years in the trade, twelve years of college and passionate involvement in the wonder and hobby which is aquatics. The following notions are intentionally generalized and therefore necessarily broad in scope and application.

It seems expedient to list some of the conditions and actions appurtenant to proper receiving of new livestock before diving into specific acclimating techniques:

1) Be Prepared: Aquaria, conditioned water, nets, tubs, and other necessary tools and materials clearly need to be on hand; including adequate personnel. If you have many bags and boxes, acclimation can take a few to many hours. The longer the livestock remain unpacked the greater stress and loss of vitality. Do you have separate quarantine facilities? What about protection of & from existing tankmates?

2) Be Programmed: Detail acclimation routines in writing and practice them accordingly. Post list(s) of what's to be done in order, label the receivers (tanks) ahead of time.

3) Be Systematic: Establish and adhere to a routine. Treat the livestock shipments in a replicable manner and keep track of your results (a small journal notebook works as well as a personal computer). Change your routines in a conscious, decided, one-variable-at-a-time manner.

4) Be Conscientious: Pick up and move your livestock from point to point without delay. Work steadily, under subdued lighting.

Freshwater Acclimation:

One last time @ this proviso: These are general methods that I've researched, tested and found to be most effective in the broad stroke. Fishes span a huge range of species, physiologies, tolerances and predictability; like people they are individuals. What has worked for me, may not work for some individual livestock or conditions...

1) On Arrival: Triage

Open the shipping containers to quickly assess whether some livestock need to be attended to first. Distressed, dead and dying organisms, obviously flat or leaking bags require immediate attention. This may not make much sense if you're a small-time hobbyist, but it will. Acquire and use a Styrofoam and cardboard box made especially for shipping livestock. It helps.

2) Measure the pH, temperature and ammonia (possibly... in general it's just too high... period), but definitely the temperature and pH to match the acclimation/receiving water's. Prepare the water to this temperature and pH (usually through the use of inorganic acids... most folks use HCl... dilute/d Hydrochloric... be VERY careful if you have little chemical/chemistry experience. This acid can/will burn your skin; the fumes are noxious to eyes, lungs... For small dealers, hobbyists, organic acids ("Blackwater Tonic"), tannins, flavones, "peat-moss" water can be employed to temporarily depress pH.

3) In almost all cases: Pour the livestock through a net and let drain. I prefer to use a bucket with a hole/fitting to drain to waste for this. It may make more sense for large specimens to be lifted from their shipping bag rather than poured through a net.

Shipping water itself is discarded to waste for several reasons. It may harbor pests, parasites and pollution. Metabolite build-up in the bag WILL negatively react with the acclimating solution; most celebratedly the change of less-noxious ionized ammonia to non-ionized formats with accompanying toxicity. Hence the matching of shipping water and receiving/acclimating water pH's.

4) Place the specimens into prepared tubs of water. My favorite containers are "kitty-litter pans" or similar; they are sturdy, chemically-inert (non-reactive), easily cleaned, soft and slope-sided to lessen impact from "bumping", easy to pick up, move and store- oh yes, and they're readily available and inexpensive.

Water Quality Water Quality for acclimation, I know there are some people who are going to scream when they read this, but I strongly endorse doing very little to alter the general water chemistry other than what is your standard management practice in your main system(s). That is, with few notable exceptions (wild Discus, cardinals and some other "wild" tetras, some S. American catfishes...) I do not alter hardness, et al. beyond what is done in the intended ultimate system. Rationale? It takes time and money and on average does more harm than good. I do try to assure that the acclimating solution is slightly warmer than the shipping water. Often the origin of the acclimation water is the outbound bagging or main system water with just the pH adjusted.


This, once again, is my own garden variety formulation for almost all types of freshwater fish livestock. Specifications are okay at approximate drops per gallon. In actual practice we re-use sixteen ounce squirt bottles of standardized-available stock solutions.

A) Eight drops of polyvinyl pyrrholidone (PVP) based dechlorinator (Mardel Laboratory's Ultra-Shield, Aquarium Pharmaceutical's StressCoat, Kordon's NovAqua...). Regarding the last product, Kordon Corporation's President, Dr. Bob Rofen was kind enough to call me on request of release of their name and trademark for this piece; he informed me that their research has shown that it is not the PVP, but other compounds in their conditioners that penetrate two to three cell layers into fishes that are responsible for their "bandaging" effects. Also thanks to Dr. Bob for urging the emphasizing that only "Pharmacy Grade" Methylene blue has the desired therapeutic properties. They reject about one out of three shipments from Europe as impure.

B) A teaspoon of non-iodized "common" salt (pickling, kosher, water-softener recharge...)

C) Eight drops of (pharmacy grade) Methylene blue.

We place the fishes into a tub of this mix and keep our eye on them. More water and/or an airstone may be added if the stock seems narcotized as if from lack of oxygen. In large scale wholesale business we have actual waterproof tables with air, treated water, drains, etc. for all this.

D) More? For several groups of types of fishes we add two more chemicals to the treatment. As a matter of availability and convenience they are Maracide (principally malachite green) and Maracyn (the antibiotic erythromycin)  by Mardel Laboratories or in some cases Furan compounds. These are routinely applied at aquarium dosage for small tetras, barbs, sharks, angelfishes, South American catfishes, pond-raised Plecos.

E) New system water of "regular" pH (typically a bit basic) is dripped into the mixing/receiving cat litter trays, overflowing to waste. Aeration via mechanical bubblers (air stones) is applied to crowded containers.

F) Removal of Dead. Statement of fact and definition.

G) Transfer. After a few to several minutes to at times hours... the entire tub is carted and dumped by overturning into its holding tank. A few notes may help clarify: Systems I've worked with run the gamut of individual tanks (non-recirculated), semi-open recirculated, to entirely open (once in and dumped); resembling everything from home use to large aquaculture operations. The chemical medications listed will rinse out or be diluted in time with appropriate water quality management (changes, filtration).

Plants and Invertebrates: Exceptions:

Most plants and invertebrates are just rinsed with temperature and pH matched holding system water and placed without further treatment. Some touchy fishes, e.g. wild discus, wild angels, cardinals are kept in a blend of R.O. water, kept warmer and "dripped" to standard conditions over a period of days.  

What Do These Acclimation Methods Do?

That is, what's the objective of all this rigmarole? First and foremost, overall to optimize the well-being of new livestock. To get them from their origination point, through the vagaries and damage of transport, to their final destination quickly, opening, inspecting, stabilizing, treating for damage, parasitic and infectious disease and "hardening" before further handling.


Rest assured I am more than acquainted with many of the other novel tools and techniques, and more toxic materials; the use of ozone, oxygen, peroxides, permanganates, copper, formalin, formaldehyde and many others. These are not worthwhile in common use as being unnecessary and dangerous to livestock and their handlers. Yes, there are other antibiotics (broad-spectrum, gram negative and positive) but Maracyn (erythromycin) and Furan compounds are effective, safe, and readily available at reasonable cost... in handy, soluble formats. Further, with careful use they don't interfere with biological filtration, and it works.

I have been party to bumping off (some euphemism now!) a few some hundreds of thousands of aquatic organisms over the last four decades in the business, science and hobby of aquatics. The above materials and methods are the best that have globally worked for me and the applications I was involved with. You are welcome to comment, add your experience, or put them to your own profitable use. Whatever system you utilize for acclimation, approach it scientifically, record your observations, and share what you learn.  

References/Further Info.

Fenner, R and Rob Lawracy. 1987. Choosing the Right Multi-tank System. PSM 4/87.

Fenner, Bob & Steve Landino. 1989. Acclimating Fishes. FAMA 8/89.

Fenner, R.. 1989. Improving Your Store With Central Filtration. Pet/Supplies/Marketing (PSM). 11/89.

Fenner, R. & Phil Farrell. 1990. An Inexpensive Aquatic Holding Facility. FAMA 1/90.

Fenner, Bob. 1991. Central Filtration Systems. FAMA 12/91.

Herwig, Nelson. 1979. Handbook of Drugs and Chemicals Used in the Treatment of Fish Diseases. Charles Thomas Publisher.

And Gratefully Acknowledging Personal Correspondence With:

Rofen, Bob. Big thanks for straightening me out as usual re Kordon/Novalek's products and various pharmacological issues. Whitney, Wayne & cohorts Denise and Fred of Mardel Labs for similarly apt-humbling with needed corrections and clarification.


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