A bit overstocked? You mention nitrate, but not ammonia or nitrite. I have a feeling there are big problems here, especially since the addition of yet more fish. Definitely run tests on all parameters. You've got, at minimum, 20 inches of fish in this tank. I'd be testing for all params. every day. If something goes wrong, no matter how small it is, the domino effect could happen very quickly. You could lose the whole tank with very little warning. Illnesses like you're seeing are usually indicative of poor water quality, and they are the first signs that things are going awry. Just be really careful -- this is definitely dangerous territory!