Is it cycled? How new is the tank?
Are you checking for water parameter issues?
Did you use live sand or
LR?
What kind of maintenance schedule are you using?
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New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?

Start here - Aquarium Advice Article
https://www.aquariumadvice.com/guide...ater-aquarium/
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves