Eh... I'm going to have to disagree with everyone here about the temperature. Goldfish, like koi, can thrive at higher temps as well as lower temps.
At 65-73 degrees, their immune system is strong. Their metabolism slows down, therefore they need less food and will produce less waste. Their lifespan may be longer than at higher temperatures, but that's still a matter of debate. If you have the option, keeping them at low temperatures is a nice low-maintenance (relatively speaking) situation.
Between 73 and 78, goldfish have the weakest immune system response. This is the temperature range at which they are the most likely to develop an illness. However, they will be more active, feed more, and produce more waste. If you keep them in this range of temps, be careful about introducing new plants and always quarantine new fish. Make sure to keep up with water changes and filter maintenance.
Above 73, up to about 83, they will have a strong immune system response, and they will be very active, but require much more food and produce the most waste. They will grow very quickly. Therefore at high temperatures it is critical to pay close attention to the water parameters, especially ammonia, and keep the tank in optimum condition.
Fancy goldfish seem to do best in the higher ranges. I see no problem with keeping this little fish in a warm tank.
Fishcatch22, I strongly disagree that goldfish kept in the 80s will be listless and sick. My five have been kept in the 70s and 80s since they were only weeks old. All are now over 11 inches long, healthy and glowing, eating like pigs, playing in the filter outlet, swimming actively. As I said, higher temps will cause more activity, not less, as their metabolism increases. Which is why at higher temperatures they produce large amounts of waste.
However, I do agree the fish is most likely stunted. We still need more information.
- What is the pH of the tank? A pH jump or drop could definitely cause illness.
- How often do you change the water? Ammonia and even low-level nitrites can sicken a fish.
- Do you treat the tap water before adding it to the tank? A low dose of chlorine might not kill the goldfish, but would definitely make it sick.
- What else is in the tank? If you have added new gravel, new decorations, etc, recently they might not be fish-safe and could be poisoning the little fish.
- What do you mean you "clean" the filter? Is it the type of filter that hangs on the back of the tank? This type of filter is difficult to keep a healthy bacteria colony living in. Before changing the media cartridge, float a fresh cartridge in the tank for a few days. This will help the bacteria to transfer to the new cartridge. If you have just changed the cartridge completely, it doesn't matter if you "scraped the bacteria" onto the new cartridge. Most of that is just fish waste, not bacteria. You may have started an entirely new cycle.
I don't mean to sound harsh, reefraft, but diagnosing an illness in an already stunted fish can be difficult. If you can provide more information, it will help. Then we can get you on the right track to providing the best home for your fish and helping it live the longest life possible.
