Can almost bet that in a tank only 7 months old with that many fish, there will be water quality issues in a 75g tank. Have you been testing over the 7 months as you add fish? Hope you got real good filtration.
My .50 cents says the LFS
will find high nitrates or an unsatisfactory level of ammonia. Phospahates could also be out of order. Bottom line, I think there are two many fish IMHO
Let us know what the water tests say. Good luck.