Fish is really happy and eating one day, dead the next

(04-23-2016, 04:02 AM)remnant Wrote: Goldfish are very fragile and slight changes in some parameters in their environment results in death. For example, they have zero tolerance to ammonia and nitrites and are poisoned by them. Other causes include oxygen deprivation associated with low pH, supersaturated gases, enclosed tank and lack of proper water circulation. Nitrate poisoning may also have caused the death or harmful bacterial infection caused by low levels of O2, improper maintenance and poor feeding routine. Finally, too much feeding, high water temperatures as well as chemical poisoning may have caused the death.

Goldfish is actually quite hardy compare lets say tropical fish. It is hard to get them killed under normal circumstances.

No fish should be in an aquarium with ammonia and nitrite (both readings should be kept at 0ppm), but they will not immediately drop dead from low concentration. So the zero tolerance is false.

Die from lack of oxygen is only likely if the aquarium is too small with very little water surface area for gas exchange with the atmosphere.

Nitrate is harmless. Although it is believed that extremely high concentration of nitrate can lower the immune system of the fish and make them easier to catch diseases and parasites. Usually we fish owners keep the nitrate below 40ppm. Only for more fragile fish (goldfish are certainly not one of them) we aim for below 20ppm nitrate. Of course the lower the better. Since nitrate is not toxic, there is no such thing as "nitrate poisoning".

Overfeeding is one of the most common causes for trouble in an aquarium, but it is an cause for trouble not the direct reason for fish's death. Overfeeding can result in too much leftover fish food in the cycle, and it can result in ammonia spike in the aquarium, and fish can
die from ammonia poisoning since ammonia is toxic.

High temperature is usually not a problem for goldfish. They can adapt to fairly high and low water temperature unlike tropical fish. Too high temperature while lack of an air pump might cause a lack of oxygen in the water.

Low PH isn't a problem either unless it took place too quickly and the PH swing is too big. Most fish can get used to the change in PH if it happened slowly.

Like I said, there are too many possibilities and there is no way for any of us to find out what's going on until the OP give more details.