Sounds like gill flukes to me. My GBR had this 2 weeks ago and the only reason I noticed was because of the flashing. Gill flukes are little parasites that live in he gills of a fish and slowly eat away at the gills. The treatment I used for the GBR was Sera Mycopur. Not sure if your
LFS sells this as the U.S doesn't allow anyone to have any products from this brand shipped into the U.S. It is a 3 day treatment that worked great for me. I think Prazipro helps against gill flukes, so if you have any of that then dose it. Treat the GBR's for it as well.
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My tanks (so far

)
5.5 gallon Planted Dwarf Puffer tank (2 Dwarf Puffers and a lot of RCS!)
16 Gallon Planted
(GBR pair, 3 amano shrimp and a whole bunch of snails!)