PeaP , I'm not a fish illness expert by no means, but I do have a suggestion for your consideration.
Large and/or extra
water changes seem to reduce stress and alleviate minor health issues before they get worse in my experience.
Fish not acting normal could be from health issues,
water parameters spiking, or could be from territorial/aggression issues for example.
Any time I notice one of my tanks with fish acting out of the ordinary, I try do do an extra water change that week, if not 2 extra.
Depending on your tank size, amount of possible muck (uneaten
fish food & fish waste) in your substrate, time elapsed from last WC, etc, consider performing a back-to-back WC. Drain 50%-75% of your tank water, refill, and repeat one more time.
Performing a back-to-back WC allows you to spend more time cleaning the substrate with a siphon gravel vac. This is especially useful when you have a smaller tank with gravel, for example 29g and under. When I had a 20g with gravel, I couldn't vac all the gravel during 1 WC.
I've switched to
sand in all tanks now, amd I've found my substrate collects only a fraction of waste compared to gravel.
My normal water changes are usually 50%-75% every 7-10 days. When time permits and its been a few extra days between WC's (or when fish are acting weird) I will do back-to-back 50%-75% WC's.
Always attempt to match the temp of new water best you can, especially when performing large WC's. Even a difference of +/- 2 °f is within range because when mixed with existing tank water, it will average out close to original temp.
I use a hose with an adapter on my sink faucet to direct fill my tanks. Makes for performing large WC's easy, especially when servicing multiple and/or large tanks.