I've had 3 tiger barbs die. Their mouths seem to get stuck open? It's not affecting any of my other community so far, but I have 2-3 more with their mouth stuck open now.. Not sure bc they still swim so quickly. Anyone have any idea what the heck is going on

50 gal. 76 degree F - 2 kissing gourami, 2 glo tetra, 4 Bolivian ram, 1 common pleco (still trying to rehome), 1 platy, 3 Molly, 2 dwarf gourami 8 tiger barb (there were 10 but I lost 2)

Can give me some idea about your filter? Which kind of filter your are using now. Give me some details about that If it is possible (means if you have the test kits) then share your water parameters.