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Thermal content and control is a mundane but absolutely critical component of aquarium gardening. It is shocking (pun acknowledged) to western aquarists that "heater cables" are so popular but such a huge expense for many of their European counterparts.
Aquatic environments, compared with terrestrial are characterized by their relative stability; particularly thermal. Yes, water is the standard for caloric content; it takes in and gives up more heat energy per unit than any other substance... and there's the rub for our small volumes. Unlike the "real world" our tanks are given to too much, too soon shifts in temperature; to the detriment of our livestock.
Excepting very large (hundreds to thousands of gallons), outside, or cool-water systems, all aquarium gardens should have a heater (and possibly chiller) and accurate way of measuring temperature. Happily, there are several makes and models of resistant commercial heaters and thermometers on the market suitable for our purposes. What's more, if you're handy and so inclined, there are do it yourself models and other ways to keep your water within a desired temperature range.
Importance of Heat:
All chemical reactions, biological and otherwise, are temperature related; too fast or slow and their is no life. Dealing with aquatic or semi-aquatic live plant systems presents a double edge sword in regards to temperature control. Natural habitats and metabolisms are subject to gradual and narrow heat changes; and water has a very high specific heat. Our controlled systems, being small in volume can get easily shifted by internal and external thermal influences; yet our livestock will only thrive when maintained over a slowly and narrowly changing range of temperature. What are these inputs and outputs?
Sources of Heat:
Think about the origins of thermal content in your system. The water itself when placed in the tank, room heating, maybe sunlight... After the set-ups running, lighting and fluid-moving pumps add "waste" heat to a system along with purposeful addition from aquarium heaters. Though not often discussed, system construction, size/shape, placement and insulation are key elements influencing thermal flow and control.
How Much Heater Do You Need? Preventative Heat Loss/Gain Measures:
The factors determining how much heating you'll require is only to a degree a matter of empirical study; for costs and safety reasons, you want to have to have marginally only as much as needed.
Tank Construction, Size & Shape
Are very important. Notably, acrylic aquariums are far superior (about five times) in thermal insulation property over glasses (for these and other materials see Sams and Webb (1979)). Regarding size, bigger is better; larger volumes contain more heat energy, and therefore more stable. Additionally, big tanks have thicker-walled, better insulating panels. Shape, for once favors my body plan; the more cuboidal tanks have less surface area/heat loss than tall, thin "show models".
Add to your reasons for not placing your tanks close against outside walls, besides mildew and not being able to work around them, the fact that those walls are energy parasites. I've seen tanks heated and cooled unbelievably by these thermal parasites. Dittos for setting the system near doors, windows and heat ducts; which leads us to:
Especially important in cool or cold water systems that you have to fight the gradient on continuously. Some lucky wholesale manufacturer will read this, and start supplying bottom, back, and possibly top and side insulation panels.
Heat loss by evaporation:
Can be extraordinary. Most of the heat leaving your system may well be from the partly or totally uncovered aquarium. If you're losing water, you're losing thermal energy. Dehumidifying air conditioning can greatly add to this.
Lights, Lighting Regimens, Pumps, et al.:
All can be guilty of adding too much heat. Consider turning on your super-nova lighting in the evenings when ambient temperatures are cooler; which part of the day, "light" is matters little to your livestock.
Though there are quartz, hot air injection, in/under tank "pad" types and much, much more, most hobbyists will use glass-walled electrical resistant heaters. These popular modern devices are comprised of a printed circuit board, heating wire, thermostat, indicator light housed in heat resistant glass.
Don't use cheap heaters! Some of the thin-walled, reception-interfering, thermostat-sticking have wiped out thousands of tanks, zapped millions of livestock, seriously injured hobbyists; and they're still on the market!. There are some reputable external heater manufacturers, but for aquarium gardens it's easier to use submersible heaters. These are inherently better because of their sealed components and ability to be placed lower in the system; warm water, like air, rises.
How much is enough? For the vast majority of systems somewhere between 3-5 watts per gallon. Smaller tanks with open tops, in cold buildings need more; big aquariums with thick walls and large thermal masses, less watts per gallon.
Placement and Number:
If you figure you need a three hundred watt heater, divide and conquer with two 150's. The rationale for this is simple, it's easier to place them distant from one another and effect more uniform heating... also, if you intend to utilize a heater chamber, or a sump, do put at least half your heating capacity in the system tank. Heaters do go out, and even though you might be the prince(ss) of elephantine memory and finesse, they "get" broken. Yes, dear friend and fellow pet-fish sufferer, I have broken (dozens) of heaters. I'll confess; some were lifted out, the water level dropped, without unplugging them. Others were stupidly immersed when too darn hot. Enough of my personal heater angst; learn from my errors. Put glass heaters low (but not undergravel) where they won't get diffed by moving decor or clumsy aquarists. Remember to Unplug/Plug in Heaters When Working on Tank.
Cable Heating Systems:
For situations where the outside temperature is very different than the water, such that the substrate is much cooler than the system water, rooted plant growth may be noticeably ill-effected. Hence a principal advantage of heating the system through within-gravel heater cables (th other being slight water circulation within the substrate). I wish these darn things weren't so blasted expensive; they're great, and really work well.
Though there are do-it-yourself folks who encourage others to build their own cable heaters, I caution you from trying to make your own. If you can't be dissuaded, at least employ a ground fault interrupter circuit with home made 110 volt circuits.
See Hamilton and Smelt for ideas on a novel approach to building your own sub-gravel heat-exchanger utilizing PVC pipe and heated water from outside the tank.
There are settings where temperature is a problem on the high end of the scale; the water's too hot. If changing your lighting regimen, encouraging cooling through evaporation... can't keep the water temperature down, you may be able to temporarily cool it by floating a bag of ice or frozen water in a plastic jug. For prolonged heat spells, you might need a mechanical chiller.
Chillers are like miniature refrigerators, capitalizing on the heat lost in controlled expansion/phase change of a refrigerant. Either an emersion coil is placed from the chiller to the system/sump, or more frequently, a heat exchanger serves as a "cold sink" to circulate system water through.
The range in quality, efficiency and average useful lifespan of aquarium chillers is huge. Be leery of all of them. If you need one, get three or more companies literature and talk with people who are familiar with their products as consumers.
Measuring Average Kinetic Energy aka Temperature:
Oh yeah, thermometers. Floating, hanging standing, liquid crystal stick-on types from a buck to several dollars are fine for hobbyist use. You don't need a meter or a high tech thermocouple; just a relatively good and consistent measure.
Are you into logging records of when, where, how much you paid for livestock, water quality tests, etc.? Good; add a column for temperature. Want another helpful piece of advice? Get at least two thermometers, and place them inside the water where you can read them at a glance, and check all daily.
Dangers in Heat Control:
Foremost, electrical shock and electrocution of you and your livestock. Heaters can suffer hair-line cracks, zapping the system slightly or big time. Inspect them frequently.
Are more fuel to the fire for keeping your system's temperature moderate. If the power goes out, don't panic. Consider applying blankets or other external insulation to your system. Floating a plastic jug, or double-insulated bag filled with warm-hot water from the tap may save your stock.
Even though they're "just" plants, aquarium gardens must be temperature controlled, keeping the water within a range and allowed to change only slowly. Hydrophytes and fish metabolisms are tied stair-step with their environment's temperature. It is eminently important that your system's heat content not change much day to day. Thermal stress is all too often a source of environmental weakening of captive livestock, leading to death from other, secondary causes.
Temperature control is vital to the success of maintaining an aquarium garden; be careful in considering thermal matters in designing and constructing your system and use an appropriate heater (and/or possible chiller).
Temperature is important. All living things live within means and extremes of heat and cold; furthermore, all suffer with sudden large thermal shift. Water has the highest specific heat and therefore resists influential temperature changes in its surroundings. Nonetheless, the small volumes of hobby aquariums can and do suffer from considerable and rapid temperature changes and should be equipped with heaters and thermometers.
Additionally, a conscientious aquarist will do their best to position their system and insulate it for warmth's sake.
Remember, all chemical reactions, including biochemical, are temperature dependent.
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