For only the third time in 20 years, I am faced with a disease (presumably) problem that needs diagnosing and then treating.
During the past couple of days, 2-3 fish have died (during the night, I find them in the morning) in the 115g tank. I did a 75% water change, followed by a 50% two hours later yesterday, but two more fish were dead this morning. Photos of a Garnet Tetra with the most visible signs are attached; a couple of the other fish were similar, but most were just pale when the corpse was found.
The area around the vent is red on the pictured fish, this is not so evident on the others. It also has a whitish lump inside the anus that can be seen. No sign of worms of any kind, so this might be a protozoan.
Ammonia, nitrite both zero, nitrate 5-10ppm which is normal for this tank. pH 6.4 and has been steady for several weeks. GH 6 dGH also steady. This tank has been running for almost a year (since changing the substrate). Foods are variety of Omega One and Hikari flake, pellet and sinking. The dead fish have mainly been hatchets, plus 3 corys, the 2 Garnets, 1 Rosy Tetra, and one headstander.