Dead X-ray Tetra

Today, after 3 and a half months,  I lost my first fish. I checked the water two days ago 0 Ammonia, 0 Nitrite and Nitrate was between 10-25.

All the fish looked fine, especially the tetra, however I just found this poor dab at the bottom of the tank. It looks like its stomach burst (see pic - sorry not best quality) though none seemed particularly swollen earlier today.

I've recently changed from flake to New Life Spectrum micro-pellets. The tetra have gone wild for it, could I be overfeeding? Or is it a disease like dropsy or other?

Thanks in advance for the help!

Thanks Sue.

These Tetra have always appeared healthy with no obvious problems, e.g. bloatedness or skin/fin problems. However, I have observed that 5 of the 6 fish always schooled well and generally stayed together most of the time, while one would often stay away from the group. Due to their almost identical appearance, it was impossible to tell if it was the same fish or different fish. It sounds awful, but hopefully it was the one that died and he always had some problem, which might mean its not a wider issue.

I will not feed today and also do my regular water change. I'll cut the amount I feed right down from now on. NLS does appear great and the fish seem to like it much more than the flake I was giving.

I will be keeping a closer than usual eye on the tank and hope there's no more casualties.

Thanks again for the advice.