fish sick ?

Question
I have 3 tiger barbs. 2 new just bought within last 48 hrs. I also have a pictus catfish. in a 2-3 week old 29 gallon tank. water quality is good , i have tested it with strips. just did a 15% water change on tues as i lost 2 tiger barbs that were bought on monday.
The catfish is fine and it was a transplant from a 10 gallon tank as was one of the barbs. I have had the one barb for almost 2 years. catfish about a year. One of the new barbs was fine this morning but when i came home from work he was laying at the bottom but would move only when another barb would chase him out. the other fish are fine just this one and i dont know why the other 2 died but one of them acted similarly . it is much smaller than the others. ANy help would be appreciated thanks

Answer
Scott,
the quick answer here is there could be a ammonia surge and the partial water change may have added to it...check the nitrates and ammonia, if the tank is cycled you will have no ammonia and some nitrates, if it has not cycled you will have ammonia and no nitrates..
If it has not cycled , don;t add any more fish and let it do what it does...when the readings are 0 or close ammonia, an slight nitrates the cycle is complete, then do a water change, 3-5 gals and add in some new fish and see what happens..