What are your goals in keeping aquatic life? Status quo maintenance of your livestock? Or are you shooting for maximized/optimized growth?
There are several important factors that affect fish growth; some may be manipulated, others not. Further is the question of whether and how important it is to drive an organism's size.
In actual aquaculture, "the controlled growth of aquatic (source) protein for human consumption", maximizing growth against cost of feed, labor, time, filtration among other costs is of premiere importance. In pet-fish aquariculture a similar challenge exists at the production/commercial breeding/rearing establishments. But what about the hobby aquarist and pond keeper? How can they, should they make efforts to optimize or maximize growth of their livestock?
Consider what you cannot change or control: 1) the genetic heritage and 2) developmental history to date of your charges.
1) Think about this; no matter what you do, your livestock will not, cannot become more or different than what their gene's will allow.
Gauging the potential of koi, goldfish, tropicals, et al. is part art, part science, and part voodoo. In the most fortunate cases, you'll be able to check out parental stock or at least members of the same population (brood, school, area) that may give you solid ideas of what might be in the way of growth rate, a maximum size, conformation (very important), color...
As an example, consider a a variety of nishikigoi (koi), the yamabuki ohgon. This is a classification of single-colored, golden metallic fish that by and large grow bigger, faster than other "types" of koi; although koi, like domestic dogs are of a single species. Also, like canines that get bigger, ohgons exhibit larger heads, pectoral and unpaired fins as young.
A similar corollary can be made for neotropical cichlids among other groups.
Important Controlling and Controllable Factors:
1) Foods & Feeding:
I've more than touched on nutrition in previous pieces. Let's briefly recap.>
(1) Feeding proper foods/nutrients, (2) in palatable formats, (3) at appropriate times and intervals, (4) in suitable quantities is critically important in optimizing growth. (5) Some foods, ingredients, natural, biological and chemical contaminants and additives have been shown to have negative effects.
What's a mother to do, feed them Total? At the more-sophisticated end, you can study up, experiment and optimize processes per the above criteria. For most of us mere mortals, frequent, smaller than possible feedings, less often than absolutely possible/practical, rotating, mixing in occasional new fresh and prepared foods is about all we can handle.
Water and Water Quality: What is this stuff anyway? Wa Wa!
This will not be an exhaustive list, but let's make some general statements concerning parameters & ranges.
There are several (from the middle English meaning many) chemical and physical phenomena that we can/do concern ourselves with at times as aquarists. Many/all of these are important in determining growth. Many of these are important in determining growth. Myriad parameters interact antagonistically/synergistically reducing/accentuating effects. My favorite and most important example is the relationship between ammonia, pH, temperature and dissolved oxygen (D.O.).> Coupling some ammonia (let's say @ 1.0 part per million) with a "higher" pH (upper 7's for freshwater, mid 8's for marine) is an invitation for disaster. Lowering ammonia and/or pH will reduce toxicity immensely. Add in an reduced temperature and/or low D.O. and you've got real trouble. Please refer to the accompanying nomograph depicting this phenomenon for koi-carp from my AKCA cohort Joe Cuny. This just-mentioned association is very likely the number one cause of captive aquatics death and hobbyist attrition.
All known measures of water quality have their optimum point, range and sustainable limit. Once again, as with nutrition, you can study up, get the best filtration system and religiously followed maintenance schedule going and more or less constantly test/monitor & modify your water quality; or be simply serendipitous (& probably more effective) and just do frequent partial water changes; geez (see biblio.). Weekly, biweekly vacuuming of gravel along with water replacement goes a long way in diluting pollutants, promoting biological filtration/conversion, and keeping the system "steady-state" (homeostatic) chemically and physically.> Make it a regular routine.
One last shot, or, okay, dig at all you tinkerers and tinkering. After twenty eight years in the trade and longer (collectively) as a passionate hobbyist, I can assure you that more livestock has been bumped-off (emphasis mine) from people fooling with their water than all other causes combined. Benign neglect goes a long way.
One final self-serving note; I do want to redeem myself as an ex-high school chemistry and physics teacher in offering a more substantive version of what water and water quality is and can be in the next few installments in this column. Keep looking.
An article of this length and scope should make mention of at least some of the pitfalls of "pushing" growth. So here goes; reasons why you shouldn't:
1) Allometry: The overall body shape, or better expressed, conformation will be different for an individual grown under different conditions and time frame. A certain desired body plan may not be easily sped up. This is so; fish achieve different shapes with varying growth rates.
2) Life Span: There is evidence that getting too big, too fast to some degree shortens fish (and other organisms') life spans. Slow and not-so-steady growth wins the race, if nothing else than by outliving the competition.
3) Color: intensity/depth and pattern may be compromised, sacrificed through accelerated growing.
4) Cost: Is the growth you're getting worth the bucks in food, feeding, filtration and maintenance?
Other Notable Factors Affecting Growth: Disease:
Infectious & Parasitic Diseases: Note that other "deviations from a normal or healthy condition have been elucidated under environmental and nutrition "disease" areas in this article. Here I want to mention the fact that infection/infestation by detrimental fungi, bacteria, viruses, protozoa et al. take their toll on growth/girth/overall index of fitness.
Social: Yes, I'm serious. How crowded with their own and other species, under what other circumstances; e.g. food, cover, temperature... can/does have growth consequences. Sex ratios, initial and widening size differences likewise are important.
As a rule of thumb, undercrowd to exacerbate growth and keep the peace. Another timely generality: maintain schooling species in small odd numbers. This seems to promote getting along and bullying. The use of "dither-fish", and mixing other species, I've more than mentioned in other pieces on various aspects of chemical communication. See them for some interesting insights into aquatic chemical/social behavior.
Ending: Thus my collective ideas & feelings on the issue. Fish do "grow" throughout their entire lives. They can "shrink" given food privation and disease. Growth can be maximized. Optimization is a different matter, taking into account consideration of costs for food, feeding, filtration among others, balanced against loss of color, pattern, body/fin conformation and possibly reproductive potential.
Do you know how to enhance your livestock's size? Should you?
Bob F. in FAMA Foods, Feeding, Nutrition of Nishikigoi (Koi Carp), Cyprinus carpio and Other Cyprinid "Pond" Fishes. 8/92 issue. Frequent Partial Water Changes. 4/89. Chemical Communication in Fishes, Pheromonal Behavior Effects, Growth Inhibiting Substance(s) of Fishes, and Schreck stoffes: Alarm Substances of Fishes, not yet in print as of this writing.