@oldhenlikesdogs Yes, I do regular water changes (weekly) and all the tanks are heated. I use the 'increase heat, put the tank in total darkness' method for velvet, with added salt and Bettafix. It does work, the fish recover within a week and never get it again. But now I don't know if they're carriers or not. Once I see any fish with signs of velvet I immediately start treatment, and I am so so careful not to cross-contaminate but somewhere I'm obviously slipping up. I feel like I just need to nuke every tank I have with treatment all at the same time. I may have to break down and try medicine tablets or something. I generally try to stay away from chemicals on general principal but I'm at that point now where I'm willing to give it a shot.
I"m just not sure what's changed in the last year that this has happened. I'd never had a fish with velvet before, ever, and my fishkeeping methods basically haven't changed so I'm not sure why I keep having this crop up now.