You need to not only get the tank healthy but you need to clean it up. Do WC
's, clean the gravel well with a gravel vac, clean the filter, clean any decorations. BUT you can't do it all at once. Do a 50% WC
while cleaning half the gravel well. Then in a couple days do another WC
and clean the other 1/2 of the gravel. The next week do your weekly 50% WC
but this time clean your filter BUT don't clean the bio-media. If it's really nasty then rinse the bio-media in a bucket of the tank water you removed, never clean bio-media under tap water or you'll kill off your beneficial bacteria. Keep working on the tank each week until it is clean and healthy and that your remaining fish have been ick free for a good month IMO
before adding anything else to the tank.