I have a thirty gallon goldfish tank with seven goldfish in it. Is this overstocked? All but two of the goldfish seem healthy, with one sometimes bottom sitting and the other with black marks that I can’t tell if are ammonia burns or not.
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You can use this to calculate whether your tank is overstocked just now or not. As a rough guide, it’s 60 cm2 of water surface area per cm of fish body length. That assumes adequate filtration.
Recommendations for large tanks with few goldfish are needed because they do live a long time and they do genuinely get big. You wouldn’t buy a dog without considering their adult size, so you shouldn’t buy a fish without considering how big they’ll eventually be. Luckily, you might have a few years and, probably (sadly), a few natural casualties before then. Not all fish live to adulthood and fancy goldfish are particularly prone to congenital deformities.
If your tank is overstocked, you’ll have problems with high nitrates and maybe ammonia spikes (which you’ll work hard trying to reduce) and a steady stream of unexplainable deaths - fish just suddenly becoming "old" when they’re really not that old. If you have the right stocking level, fish keeping is much, much easier. I hope you can reduce your stocking level somehow as they grow (bigger tank, or a part exchange, some fish shops will part exchange bigger fish that have outgrown their tank).
As for the black bits, could be a juvenile. Babies often have great markings that disappear with age. And some goldfish (my own included) sleep on the gravel. They literally just bed down and go into a suspended animation. Check the fin and swimming position when it’s up and about. A healthy fish will have its fins held out and up and proud. An unhealthy fish will clamp its fins. Some of them simply don’t make the traumatic transition from breeder to shop to home.
