I had 6 cordydoras napoensis in a 60 Liter tank, together with a small number of neon tetras (6) and guppies (5) and a handful of baby ramshorn snails. Yesterday, one of the cories mysteriously died. A day before they had trouble swimming straight and got lethargic. The other fish do not seem to have any visible issues.

I could make out no obvious symptoms of known illnesses on the dead cory. The barbels were intact, there was no noticeable bloating or thinness, they had no unusual white, red or black spots. There was a tiny red spot on the chest area, but very faint and not comparable to red blotch disease. I used test strips to confirm the water is as always, <20ppm nitrate, 0 nitrite, 0 chloride, about 7pH, about 6° CH and >6° dGH. I change 15% of the water every 2-4 days, and treat the new water with detoxifying conditioner.

I did not change anything in the tank recently either. The most recent change was to the diet. I purchased microfex worms (dero digitata) and am culturing them in a separate container. I change 90% of the water every 2 days, and the worms that end up in the sieve go in the fish tank as live food. Apart from that I feed about 0.1 gram of pellets every day, and maybe 0.05 grams of fast sinking pellets for the cories. On the weekends they get a small JBL NovoTab wafer.

I have a quality external filter running on the fish tank. There are several plants, some catappa bark, some wood, a heater keeping it at 24-25° C and an aerator. The substrate is small, smooth gravel and a few patches of sand.

Any ideas? I want to prevent that from happening again, but I don't even know what happened.