My fish will flash after a large wc sometimes. If I turn up the sand they itch from little bits gettig on them. If I do a really large wc a little late they flash because there is a ph change. My tap water is ph 7.5 but after being in the tank it drops to 6.5. So that will do it. If I only do a little wc it's no big deal orlike with my discus I do a wc everyday so the ph does not get a chance to drop that much.
Is the flashing just a little after wc and then they stop? Or is it something you see happen everyday?