Do fish grow only as large as their genetic capability and physical environment allows? Well, sort of. Metabolite effects, are often more rate/size limiting. Say Whaaa?
Under my files of least favorite "here we go again" pet-fish urban myths to re-hash and debunk is, "are there growth inhibiting/stimulating substances?" Yes, yes, and yes, and yes! they are important and interesting as all heck. Let's tackle what G.I.S.and G.S.S(ubstances) are, what they do to and for your wet pets, what you can/should do about this new or reinforced knowledge. Not enuf angst? Maybe next we'll tackle whether to float new introductions or not. (Hah!) Read on Bartholomew.
En media res (in the middle of things, eh?) Introduction:
Geez, it just seems like (yesterday) the latter 60's and 70's the topic of whether fishes, and other living critters, plants, algae... produced chemicals that influence positively and/or negatively their growth (reproduction, color,...) was being heavily disputed, even disparaged. Allow me to call-up the names of some of my hobbyist bulletin writer-heroes, Jim (the hammer) Langhammer and Kappy Sprenger, among others from those times to present for clear & fun cerebral activity on this and many other issues.
All living things, by definition, engage in biological, read that as bio-chemical/physical reactions transforming some materials, utilizing energy and producing "waste" and growth products and so-called by-products. The various chemicals changed and passed outside particular species may be classified in the following way:
1) Having no specific/known effect(s) on the secreting species or others.
2) Having such effects:
A) On the species itself (pheromones)
B) On other species (allomones)
What is this Stuff, Anyway?
Many exocrine (versus endocrine) substances have been isolated/identified to have G.I.S. and/or G.S.S. effects. Ammonia-derived metabolites, amino acids and combinations of A.A.s, and most notably short chain fatty acids, have been demonstrated to accelerate/retard the individual growth and behaviors of many groups of fishes.
Most "advanced" (old and wrinkled?) aquariologists know the practical whys and wherefores (consequences) of these substances. Some highlights:
1) Everything else being equal (that is, no food, gas, physical space, filtration, lighting, temperament, other limitations) some same-species fish substances:
A) restrict spawn sizes, differential growth rates of individuals and groups of individuals, further:
B) limiting their subsequent reproductive viability, and even:
C) the smaller (runt) individuals survival rate (demise).
2) Mixing species often lessens the pheromonal stunting effects of a single species alone.
3) Boiling the water, leaving it to age (for weeks), dissipates the responsible compounds.
4) Removal of some or all of the larger members of a spawn or population spurs the growth/development of the next larger individual(s).
5) Some of these materials have beneficial and antagonistic impacts. That is, using "old water" has shown healing and growth-positive actuation compared with absolutely fresh (i.e. no-same-species-chemical secretions) water
6) That frequent massive to partial water changes and effective chemical filtration lessen these effects.
Why? Tell Me Why?:
Towards what possible ends would/should organisms produce such self-limiting secretions/excretions. Several reasons come to mind: 1) To preclude over-crowding and loss of the whole population due to food, oxygen/carbon dioxide limitations, other controlling factors, in the otherwise absence of other species competition or predatory pressures. And I'll leave it up to you to decide whether this one should have been mentioned first, 2) the ole Chucky Darwin Natural Selection (long may it wave?). By having the most humongous citizens beat out the less-humongous as a gauge of "fitness", may enhance the overal survivability of the species.
Where's this Stuff Come From?:
Principally growth stimulating/retarding substances have been found, and presumably are released by way of, body slime, excretory discharges from the anus and gills, and traumatic damage to the organism in general. This last category will explore in our next visit as Schreckstoffes: Alarm Substances of Fishes.
Closing: (Sort of):
So... What's an average pet-fish-ichthyologist to do, anyhow? Mainly, not worry too much; at least not more than their benefiting from their aquatic experience. Actually there are (to my understanding) only a couple or three "things to do" to ameliorate the cumulative negative effects of metabolite build up:
1) Try to selectively filter/change these compounds. Various carbons and clinoptilolites (e.g. zeolite) have showed various positive results in removal of ammonia salts (see Konstantinov et al. re Cyprinus carpio (koi) and Brachydanio rerio (zebra danios) experiments). Still gotta plug skimmer/protein skimmers, with or without ozone et alia embellishments.
2) Serial dilution through, Yes, my favorite: FREQUENT PARTIAL WATER CHANGES; Yay! Certainly the best, least expensive, effective means. Oh, semi-lastly,
3) Flood the system with complementary chemicals. Live plants, a mix of communities of macro and micro-organisms, the whole "life'juice" of the system... the more complex and complete, larger, the better. Live plants, algae, generally all organisms investigated also have their phytohormones et al. affecting/influencing their own and other species. The more these are integrated and functioning, the more naturally homogeneous and self-stabilizing (homeostatic) your system will be. Therefore the argument for linking your tanks together, use live plants, live rock, etc, and finally, lastly
4) The usual harangue about mis/over-feeding. The more glop tossed in, the worse. Feed sparingly, at correct intervals, of useful foods. Are you feeding for growth or maintenance or what?
Biblio. et More: Mainly hobbyist inclusions for historical reasons.
Anon. 1988. Stunt Work. T.F.H. citation of Daniel Heath and Derek Roff, "Test of Genetic Differentiation in Growth of Stunted and Nonstunted Populations of Yellow Perch and Pumpkinseed". Transactions of the Am. Fish. Soc. [116(1):98-102]
Drickamer, L.C. Pheromones: Behavioral & Biochemical Aspects. Adv. Comp. Environ. Physiol. 3, 1989, pp.269-348.
Fenner, Bob-O. 1989. Frequent Partial (what else?) Water Changes. FAMA 4/89. Some self-aggrandizing citation now!
Konstantinov, A.S. & M. Yu Pelipenko. Use of zeolite to remove toxic substances from nitrogen metabolism of fishes. J. Ichthyol., vol. 23, no. 6, pp 159-161, 1983.
Langhammer, Jim. 1976. G.I.S. - G.P.S. - Optimum Crowding, A Possible Synthesis. Tropic Tank Talk. Various issues during the year.
Sprenger, Kappy. 1974. Growth Inhibiting Secretions. Colorado Aquarist. Jan. 1974. Reprint of the original from San Francisco Aquarium Soc.
Stacey, N.E. Role of hormones and pheromones in fish reproductive behavior, An evolutionary perspective. Prentice Hall, Inc. Englewood Cliffs, N.J. 1987, pp. 1-350.