How fast do fish grow?
PFK often talks about tankbusters and how fish can easily outgrow their tanks. But just what should I be expecting regarding growth rate? I have three clown loaches in my 500-litre community aquarium. They were
3-4cm in length when I bought them three years ago. In this time, despite being healthy and doing well, they have only put on a centimetre or two.
Clown loaches are often put into the ‘tankbuster’ category with an eventual size of 20-30cm. But at this rate they will outlive me before they get that big.
Is there any truth in the old fishkeeping myth that fish grow to the size of their tank? I’d be interested to hear your thoughts.
HOWARD DAVIES
NEALE SAYS: Growth rate of fish is incredibly variable. To a large extent, it’s based primarily on adult size and longevity. Annual killifish species that never reach more than 2-3cm in length will need to grow rapidly, often reaching sexual maturity within a couple of months. Livebearers like guppies that, in the wild at least, have short lifespans grow similarly quickly.
On the other hand, a fish like an oscar that easily lives for 10-12 years takes a little over a year to become sexually mature. Clown loaches certainly fit into that bracket too and are notoriously slow growing compared with many community fish.
On top of that, males reach maturity more quickly than females—which can be attributed to the greater metabolic costs involved with producing eggs compared with sperm—and may never reach anything like the size of fully grown females. Taking guppies as an example, the males are fertile at around six weeks of age, whereas the females are unlikely to become pregnant much before they’re three months old. Adult male guppies are, of course, barely half the size of female guppies, which again seems to be adaptive. For males, they’re expected to ‘live fast and die young,’ with their brightly coloured tails attracting predators as much as females. Any males that do survive long enough to breed will have demonstrated their genetic fitness just by doing that, making them attractive mates. The females, by contrast, are bigger, so less vulnerable to predators anyway, but also less brightly coloured, so they can hide away better as well. Given they’re carrying baby guppies inside them, the better the females are at surviving, the better the chances are they’ll safely deliver their babies when the time comes.
But there are other factors that regulate how quickly a fish grows. The environment is one, and fish farmers have found that some fish species, such as black pacu and tiger pufferfish, grow more quickly when stocking density is low, but other fish species, such as
Tilapia, don’t seem to be affected, and provided diet and water quality are good, will grow well regardless of stocking density.
Food is also a major factor, particularly when rearing juveniles. If a fish is starved or otherwise kept poorly when very young, it might never ‘catch up’ with its peers, however well it is subsequently maintained. Such fish may be termed ‘stunted,’ but apart from a smaller than usual size, it might otherwise be in good health.
Yet another factor is genetics. In the wild, selection pressures favour particular sizes. There’s usually a balance between pressure towards
smaller sizes—which allow more individuals, and therefore more genetic variation, within a given area—and larger sizes, which allows for better outcomes in territorial disputes and reduced vulnerability to predators. Whether a species evolves towards a smaller or larger size will therefore depend upon their particular situation. But in captivity, where predators are absent and food is virtually offered on a plate, there’s no real advantage to being big. Indeed, fish breeders will tend to select fish that become sexually mature relatively quickly, at smaller adult sizes, simply because the first fish to start spawning are often the ones taken to the breeding tank. Fish that grow slowly, and become sexually mature relatively late, may still be bred, but given the same lifespan as their smaller siblings, may well produce fewer offspring. So, over time, smaller adults leave more descendants, and the gene pool shifts towards a smaller body size and earlier sexual maturity.
Provided diet, water quality, and social behaviour all seem fine, I’d not worry too much about your loaches seeming to grow more slowly than you might have expected.