This sounds strange. Can you get a few pics of different groups in the tank?
What are all the parameters, GH
Any thing - changes, like moved the tank, changed media, foods, ferts, light schedule, temp of tank? Water change schedule difference?
Plants, moss, new shrimp, fish, snails?
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves