Flashing, red gills, black spots on fins, not eating, death are a few
Water changes are the answer.
If you are cycled there shouldn't be any in the tank
weekly water changes to keep everything in check.
check your levels because I just went through a mini cycle in my tank due to API root tabs.
I believe my angle and dwarf gourami had it because they both started flashing their gills on my VAL plant. a couple water changes some stress coat and my levels are back to normal and its subsided.
I saw my angle flash last night once so maybe it still a little burnt.
Pretty sure angels are over sensitive fish.
Maybe throw some safe start in the tank to be safe?