Growth and development of aquarium fishes.
From a fertilized egg, a larva develops, which differs from adult fish in body shape and often in the presence of various temporary organs. Depending on the amount of yolk, the yolk sac remains in larvae for a more or less prolonged period. Upon completion of its resorption, the larvae of most fish acquire the appearance characteristic of adult individuals of this species; they switch to active feeding and turn into fry. In some fish species, this process is somewhat delayed.
Already at the larval stage, linear and weight growth of the fish begins. During this period, growth occurs mainly due to the resorption of the yolk sac. With the transition to active feeding, growth continues due to food. Fishes grow throughout their lives – initially this process is rapid, then growth slows down increasingly.
In addition to the age of the fish, its growth rate depends on a number of factors. The quantity and quality of food are of great importance; most fish at earlier stages of development need a large amount of relatively monotonous food, while for many adult fish, a change of diet is desirable. Temperature plays an important role; as a rule, the higher the temperature, the faster the growth, reaching a maximum under optimal conditions for a given species and slowing down with a further increase in temperature, and then completely stopping. Hydrochemical conditions also affect the growth rate of fish. Not least is the species identity of the fish (fish of different species grow at different rates under optimal conditions).
A combination of all favorable factors is important. Thus, optimal temperature and oxygen conditions without the necessary amount of food or the specified factors under unfavorable pH conditions will be insufficient for normal growth. With various combinations of factors influencing growth, there may be a discrepancy between the increase in linear growth (length) and the weight of the fish.
Development begins from the moment of egg fertilization and is divided into two periods – embryonic and post-embryonic; the latter begins after the larva emerges from the egg membranes. For normal development, basically the same factors play a role as for growth.
However, the significance of individual factors for the growth and development of fish in some cases does not coincide. Thus, under certain feeding, normal growth can occur, but the development of the reproductive system is delayed or completely stops (as a result of obesity). And, conversely, more rapid maturation is often observed with delayed growth.
Upon reaching puberty, the gonads finally develop, and the hormones secreted by them cause the manifestation of sexual characteristics. Based on these features, males are distinguished from females in most cases. Primary sexual characteristics include the copulatory organs of male livebearers and Cichlidae, and the ovipositors of female Cichlidae. The development of their gonads with the age of the fish often causes the appearance of secondary sexual differences in body size, fin size and shape, very often in the degree of brightness and nature of coloration. Many secondary sexual characteristics are manifested to a much greater extent during reproduction; some – only during the reproduction period, this is the so-called breeding dress.
Most aquarium fish live no more than 3-5 years, some Cichlidae – over 10 years, goldfish – several decades. Some species of egg-laying toothcarps, which live in natural conditions for only a few months, live for several years in an aquarium. It can be considered that in aquariums, fish, as a rule, live longer under favorable conditions than in the wild. Here they are devoid of competition and receive a sufficient amount of food. In fighting fish, it has been noticed that they live significantly less when frequently placed for spawning.
Does the size of the fish depend on the size of the aquarium?
I am sure that each of us has heard the statement that the maximum size of a fish depends on the size of the aquarium in which it lives. This statement is based on the fact that a fish will never be able to become larger than the aquarium in which it lives, and will not even approach the size of the aquarium. And this is indeed true. However, does this mean that the first statement is also correct?
I will try to explain why, in my opinion, this is not entirely true. First, it is completely clear that the maximum size of a fish is genetically predetermined for each fish species. In addition, it must be taken into account that a fish grows without stopping until the end of its life (faster at a young age and slower at an old age). That is, the maximum size of any fish species is initially determined and should not depend on the size of the aquarium where it is located. On the other hand, it can be noticed that in large aquariums, fish grow faster and reach larger sizes than in small ones.
So does the size of the aquarium affect the final size of the fish or not?
I believe that the size of the aquarium can affect the growth rate and lifespan of the fish and, as a consequence, the final size of the fish due to the fact that in small aquariums the fish does not have time to reach its maximum size for a rather sad reason – it dies long before that.
Let’s consider how lack of space affects the growth of a fish.
1. In a small aquarium there is little space for the movement of the fish and, just like in humans, as a result of hypodynamia (lack of movement), various diseases associated with it appear, ranging from obesity to cardiovascular diseases. As a result – poor health of the fish and its premature death.
2. In a small volume, it is harder to maintain cleanliness and the necessary water parameters. Food residues, feces, etc. quickly lead to sharp changes in water parameters and its pollution. There are jumps in the level of ammonium and nitrites, in water acidity. All this leads to excessive stress in the fish and its weakening, and as a result, to fish disease. The result is poor health of the fish and its premature death.
3. In a small volume, the level of nitrites, nitrates, and other toxic compounds quickly rises, which negatively affect the health of the fish, especially the respiratory system, kidneys, and liver. Disruption of kidney and liver function leads to fish disease and, consequently, to premature death of the fish. Damage to the gills leads to poor oxygen supply to the fish’s tissues (hypoxia) and, as a result, poor fish growth, weakened immunity, and premature death of the fish. In addition, it is known that high levels of nitrite and nitrate cause delayed fish growth.
4. In small volumes, with a lack of swimming space, the fish can receive mechanical injuries from various decorations and damage scales, skin, and even cause skeletal deformation. There is a risk of infection with secondary infections and, as a result, poor health of the fish and its premature death.
5. Poor or unsuitable water parameters for the fish, constant stress can lead to loss of appetite. A fish that eats poorly and receives insufficient elements necessary for growth and health, understandably, grows much slower. In addition, a fish that eats poorly weakens, its immunity also weakens, and the risk of infection with various diseases increases. The result is poor health of the fish and its premature death.
Conclusion – when keeping fish in small volumes, there is a greater risk of diseases and death of fish. Based on all of the above, I believe that the final and maximum size of the fish does not directly depend on the size of the aquarium. That is, if even in a small volume we create good conditions, monitor water cleanliness and the necessary parameters for the fish, then the fish will grow well and quickly, and it may happen that we witness the aquarium becoming small for the fish. It is known that in an aquarium with good filtration and frequent water changes, fish grow faster.
Do a simple experiment – divide a large aquarium into unequal but interconnected parts (large and small), and put fish of the same species in each section. The same feeding, the same water parameters – the same growth rate in both groups of fish. That is, the growth rate and maximum growth of fish in an aquarium depend not so much on the size of the aquarium as on the conditions in which the fish live. True, these conditions are easier to maintain at an acceptable level in large volumes than in small ones.
Very often, when keeping fish in a small volume of water, they begin to grow unevenly, so much so that specimens that have outgrown others begin to eat their stunted 
congeners. In an overcrowded aquarium, there are all conditions for an outbreak of severe infectious diseases. In a limited volume of water, fish are more susceptible to various diseases, depressed, and offspring from them, as a rule, cannot be expected. In a small aquarium, it is cramped for active fish (such as barbs, for example). And it is completely cruel to keep fish in a small volume that by their nature should grow to large sizes (goldfish, large Cichlidae, angelfish).
It is also very important to select fish of approximately the same size to avoid losses or developmental delays in smaller specimens. But in the case where Cichlidae have different characters or degrees of territoriality, relatively “peaceful” fish can and should be chosen larger than more aggressive ones. The sizes of the fish and the aquarium must correspond to each other. The larger the aquarium, the better decorative effect can be achieved and a large number of decorative fish can be well accommodated. In any case, the sizes of the fish and the aquarium must correspond to each other. Therefore, from the very beginning, it should be clear which fish species you want to keep and adapt the choice of aquarium sizes to the most demanding species. If the aquarium is already available, then in this case the fish species are selected according to its size.
Fish sizes in ichthyology and aquaristics.
Revir – the volume of living space of fish. 
