not sure about the rest of the fish the first time around but ur clown could have reacted to the treatment u put in , my hubby put treatment in a few weeks ago the clowns went crazy swimming so fast they were slapping into the glass , had to do a emergancy water change ..soon after they were fine ..so that could be the reason ,,,,,,,,, and for ur missing fish , the other fish /clowns would have had a feast ..
check your water is fine , do u test with a liquid test kit? if not a option is to take a water sample to nearst lfs
cleaning ya filters is a good idea , like ya sed one this week maybe the other cpl weeks later ......
but its hard to just without water stats ..
any visible signs of fish flashing at all ? mine flash if water is off ..