[i have crabs]

lots of carbon and waterchanges

as for the qt situation it could have been a few things.
1. its a new tank and will cycle like a new tank, so adding a fair size fish into a 10g could have made an amonia spike
2. you said he was hard to catch, im no doctor but when fish get scared/stressed from something like being chased by a big net they build up acid in there blood. lactic acid maybe? cant remember the exact details with out looking it up. anyway after being chased around like crazy the acid kills them shorty after.


and i still advise you to look up some articles on ich, maybe from reefkeeper magazine online,
only putting 1 fish in the qt and treating them will kill the ich on the actual fish which will take closer to a week, and its recomended to treat for 2 weeks, but without treating all the fish and leaving the tank bare for a few weeks your not completely getting rid of the ich just the symptoms,

other red flags that hit me in the face are amonia/nitrite showing up at all, and the other fish having cloudy eyes which sounds like a bacterial problem, it sounds like your doing waterchanges which are good but something dosent seem right to me.

are you using tap water or ro/di?
how long are you mixing up the water before adding it to the tank?
maybe do you have some pics of the tank?