A lot of different factors act together to determine the growth rate of your fish and how large they will become. Some of these factors are constants, e.g. the species or your fish, while others can be adjusted by the aquarist to ensure optimal growth rates. In this article we will take a closer look at constant as well as alterable factors of imperative importance. A word of caution before we proceed: making a fish grow really fast is not necessarily a good thing. What we want to optimize as aquarists is the healthy type of growth that can be achieved without doing anything to the fish that can prove detrimental in the long run.
DNA
The DNA of each fish specimen will always put a limit to its maximal growth rate. Tinkering with the DNA of each specimen cannot be done by hobby aquarists; the only way to affect the DNA is to participate in dedicated, long term breeding programs where desirable genetic configurations are encouraged and undesirable ones are removed from the breeding stock. You can also make sure that you purchase your fish from reputable suppliers.
History
When you purchase a fish, it will already have a long history that affects its maximal growth rate. A wide range of factors are at play here, from the overall condition of the parents and the situation in their breeding aquarium to the water quality of the fry raising aquarium and the diet given to your fish in its crowded pet store holding tank. If you want to ensure optimal growth rate you should therefore stick to reputable breeders and pet stores that you know take good care of their fish.
Foods and feeding
Your fish needs a varied diet that contains all necessary nutrients in suitable proportions. Starvation as well as overfeeding (which leads to obesity) will both have detrimental effects on the growth rate of your fish. Fish need to be fed suitable amounts of food at appropriate times and intervals. When picking food for your fish, it is naturally important to stay away from ingredients known to have negative effects on fish health and fish growth.
If you fish stops eating or eat very little, you must always try to find out the reason why. It can for instance be caused by disease, improper choices of food, a monotonous diet, bullying tank mates, or improper water chemistry and/or levels of nitrogenous waste. Sometimes simple changes such as giving your big cichlids large pellets instead of miniscule flakes will solve the problem. In other situations, you may have to entice your fish with live food to trigger their hunting instinct and make them resume feeding. Learning more about how, when and what your fish eat in the wild will provide you with valuable information about how to provide them with optimal food in the aquarium.
Water
Generally speaking, the most important water parameters in the aquarium are temperature, pH-value, water hardness, and levels of ammonia/ammonium, nitrite and nitrate. These factors are known to interact with each other and altering one of them can produce surprising effects on the other ones. It is therefore important to do your homework before you start fiddling with the water conditions. One thing that you always should do is to strive to keep the levels of ammonia/ammonium, nitrite and nitrate down by combining filtration (biological and/or mechanical) with regular water changes.
The only way to know which water parameters that will ensure optimal growth rate in your aquarium is to research the species you are keeping. Avoid mixing fish species that require highly dissimilar conditions since they will force you to keep the water parameters at levels that are “least bad” for all species instead of optimal for species from a certain biotope.
Disease
Disease will force your fish to spend a lot of energy fending off malicious microorganisms and there will be less energy left for growth. This means that even if you know that your sturdy fish is capable of handling Ich or dealing with velvet, you should still strive to never introduce these parasites to the aquarium in the first place, since disease will always steal energy from your fish that could have been used for growth. Frequently using medication to remove disease from the aquarium is not the right course of action because being heavily medicated is also stressful for your fish and will hamper its growth rate. Prevention (always quarantining new fish, cultivating your own live food, soaking new plants in saltwater etcetera) is therefore strongly recommended.
Tank mates
The choice of tank mates is very important, because bullying tank mates will cause stress in weaker fish and this can have a detrimental effects on the growth rate. Overcrowding an aquarium is also a bad idea in most cases, but there are instances where aggressive fish will fare better in heavily stocked aquariums since it will spread out aggression over many individuals and prevent territories from being claimed. Schooling species should always be kept in schools, since they hate to live alone.
Potential pitfalls
An abnormally fast growth rate can have detrimental effects on the overall health and appearance of your fish and should therefore not be encouraged. An abnormally fast growth rate can for instance produce an undesirable body shape, e.g. a fish that grows very long but not sufficiently tall. An extremely fast growing fish can also fail to develop its sought-after colors (including color intensity and depth), patterns, elongated fin configurations and so on. Last but not least, there is evidence that growing too large too rapidly can shorten the life span of a fish.
