First off, what are the water parameters?
Maybe your parameters are not within safe levels?
Anytime there is a fish death, do routine pwc
or two in the following days. It is just a normal task to do after a death.
Fin errosion can also be a sign of water problems.
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