Looks like he got sick from very bad water quality, and judging by this picture it looks like you need to seriously work on your maintenance schedule before adding anything else to that tank.
I would do a thorough cleaning and vacuuming of everything including filters, decor, equipment and especially the substrate. Be careful so you don't destroy the beneficial bacteria in the filter media if there is any at all, and please put the fish in temporary containers while you stir the enormous cloud of ammonia and decomposing organic matter that is surely living in that neon colored gravel lol
Good luck with your future projects!