▷ Fish Growth and Fish Size | All info

What factors determine how big a fish gets

On the one hand, the size of a fish is genetically determined at the species and individual level. The final size is then influenced by certain environmental factors. Genetics speak of genotype and phenotype.

It is not clear which environmental factors play a role. The following factors may play a role:

fresh water supply

In one experiment, normal carp were placed in smaller tanks and fed well. Uncontaminated fresh water was permanently routed through the basins. The carp got so big they practically outgrew the tanks.

Experiments with hourly water changes are said to have proven for numerous fish species that fresh water supply is crucial for growth.

A connection with nitrate pollution is therefore not proven. It is suspected that an as yet unknown hormone or something similar is secreted, which acts as a growth inhibitor when there is a higher organic load in smaller tanks or due to infrequent water changes.

It should also be proven experimentally that constant water changes also promote growth when nitrate is also added.

It is possible that the germ pressure is not so high due to the frequent supply of fresh water, so that the fish need less energy for the immune system and can invest more energy in growth.

nitrate content

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Corydoras paleatus also grew to sizes of 6 centimeters and more in small tanks when the nitrate levels were very low.

In many aquariums, the nitrate levels are relatively high at around 25 to 50 or 60 mg/l. It is possible that nitrate is a growth brake and has a growth-inhibiting effect even at these values.
According to the German Drinking Water Ordinance, a maximum of 50 mg/l nitrate in drinking water is permitted, according to the Swiss Water Protection Ordinance only 25 mg/l.

Water with higher nitrate levels is not recommended for feeding infants because of the risk of nitrite formation from bacteria in the gut flora.

The different limit values ​​show that it is disputed at what level there is a possible risk to health.

The limit value of 50 mg/l nitrite in tap water is probably a compromise, because otherwise many water suppliers would have given up their wells or at least had to make significant technical upgrades. Or they would have to blend more with low-nitrate water.

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So that fish have optimal conditions, especially for breeding, combating nitrate in the aquarium water is possibly just as important as using reverse osmosis systems, etc. The nitrate content can be reduced primarily through good plant growth. Plants such as ivy, monstera, etc. can also be helpful.

However, nitrate cannot be the only factor that has a decisive and inhibiting effect on the growth of fish. Otherwise, fish farming in so-called recirculation systems would be difficult. There are investigations in which no harmful influences on the fish could be proven at nitrate values, which are absolutely unacceptable for aquariums.

Perhaps nitrate is more of an indicator of water pollution. If there is a lot of nitrate in an aquarium, there is a high probability that there are also high concentrations of other substances. Possibly also substances that aquarists cannot detect with their normal means, eg hormones, pheromones, etc. Then nitrate may only indicate that something is wrong, but it may not itself be the cause of the problem.

hormones

Hormones are substances that are subject to a strict regulatory cycle, often with multiple safeguards. Deviations from a target value, which is intended to be corrected, are decisive for hormone production. What triggers this deviation depends on the hormone and is therefore very different. A control circuit in which the pool size is the setpoint generator is difficult to imagine. At most, factors that depend on the tank size are conceivable, such as stress due to excessive stocking, pheromones, fluctuating or too stable water values ​​or perhaps nitrate.

At least one theoretical possibility would be the elimination of growth inhibitors in order to keep the competition, i.e. the siblings, small. This could explain several observations. All measures for better water hygiene also dilute these hypothetical substances, water changes, better biological filtration, more space, UV lamp etc. If the concentration of these substances is lower, the fish grow faster.

Such substances have also been discussed in plants.

swim room

It is repeatedly found that adolescent fish, eg guppies, which are transferred from a small aquarium to a larger aquarium, have a significant growth spurt compared to their siblings who remain in the small aquarium.

According to one report, Copella cf. nattereri in an aquarium in the small area behind a mat filter with a size between 10 and 25 millimeters were significantly smaller than younger and same-aged conspecifics in the same aquarium, which had more swimming space available because they did not get into the narrow area behind the mat.

On the other hand, Nicaraguan cichlids from a brood in a 60 cm tank with 1/3 water changes every other day grew significantly faster than the siblings in a large tank with a 1/3 water change once a week.

There was no competition for food from the parents because the parents were taken out of the aquarium beforehand. Nitrate was not detectable in either aquarium.

This experiment should show whether fish adapt to the aquarium size. The experiment suggests that if fish aren’t growing properly, they’ll fret. However, this does not necessarily have to do with a lack of swimming space, but that other factors also play a role.

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water flow

According to one report, two 250 liter tanks were set up identically and guppies were used for breeding. Both pools were filtered with an air lift mat. However, a flow pump with 1500l/h was installed in one pool. Both pools had no substrate, 2 bunches of Vallisneria in flowerpots, tap water. Water changes are carried out simultaneously and with the same amount of water.

The guppies in the current tank grew significantly larger than the guppies in the other tank. The males in the current basin were about 5 centimeters tall, the females about 8 centimeters. The males in the other tank were about 3.6 centimeters, the females about 5 centimeters tall.

The guppies in the current tank grew faster and were fitter. They showed no signs of illness, although the stocking was significantly higher than 1 centimeter per liter of water.\\ Ichthyo appeared several times in the other tank. After a 1000 liter pump was installed there too, no more ichtyo occurred there either. In addition, all the guppies got a growth spurt and became almost as big as the guppies that were in the current tank the whole time.

The most unfavorable of several factors determines growth

Fish growth cannot be attributed to just one factor. Water could be changed like crazy, yet the fish would not grow unless fed.

The guppies had less swimming space and less food. This kept them smaller. The Nicaraguan cichlids had less swimming space, the same food, but more water changes. This made them bigger

Similar to plants, the worst of all factors determines growth in fish.

Society is also an important factor. This also has an impact on the swimming space, but also includes dominance behavior and competition among each other.