Corydora problem

Question
Hi, I have a 10 gallon freshwater tank with four corydoras. The tank has been running for close to 10 weeks now. During the break-in cycle, I got the first two and then when the tank cycled, I got the last two. I was pretty diligent about water changes and making them comfortable during that time. The ph is now 7, and ammonia/nitrite levels are 0. Nitrate levels are very low. These levels have been this way for over a month.

Within the last couple of weeks, one of the corys appeared to be sick because it was listless, not eating and was beginning to isolate itself from the others -- sometimes hiding. I could not see any visible signs of disease until last week -- it seemed to have a balancing problem. It died today. So, my question is, what could have killed it and how do I protect the others? They are eating and are more active. Could the fish have been weakened during the cycling period and succumbed to some parasite or internal bacterial problem? I might add that it was never an enthusiastic eater.

Thanks for any suggestions, in advance.

Answer
Hi There,

It is possible that your fish succumbed to disease whilst your tank was still in the cycling process. High ammonia levels are present in a newly set up tank which are toxic to fish.
The balancing problem sounds like swim bladder disease. Usually fasting the fish for 2-3 days and then feeding frozen shelled peas will help with digestion.
To make sure your fish stay healthy i recommend frequent weekly water changes.

Hope this helps,
Chloe :)