Hello all, I have posted on this issue a couple times but now I am at a complete loss. In my 20 long I had 10 pygdmy corys, 3 peacock gudgeons, 8 harlequins and a peaceful betta. Tank is established and has steady reading of PH 8.0, Ammonia 0, nitrite 0, nitrate below 5 ppm. Water is slightly hard but nothing crazy. The pygmy's began rapidly dying off, betta suffered major fin loss (not fin rot) and my gudgeons were flashing like crazy. This started about 3-4 months ago and during those months I have (separately) dosed API General Cure, Furan-2 and Kanaplex, back to back Prazipro treatments and then Paraguard. The pygmy corys all died pretty quickly making me think I had aggressive columnaris, my bettas fins are now slowly regrowing, however my gudgeons continue to flash fairly frequently and every once in a great while I'll catch a harelquin flashing as well. I do 30% water changed twice weekly. I am just totally and completely stumped as to what could still be causing them irritation.