Help - green tiger barb deaths

We (live-in boyfriend and I) have a two-gallon with one green tiger barb. This is the first barb we got. First we had him in a five-gallon with three tetras, but he was too aggressive, so put him in the two-gallon. We then got two more g-t barbs; they died after about a week. Water tested ok. Got one large barb (female I guess?) - the original barb seemed very happy with her -- stayed close to her and seemed to be nuzzling her; saw no aggressiveness or nipping. She died within a week. Water tested okay. Got a very small g-t barb (thinking it would acclimate better if very young); saw no aggressiveness by the original guy - new guy didn't last two days. Any idea why the original barb has survived, but every one since has died? (I use treated tap water and replace about 25% every two weeks - use Stress Coat each time and when adding new guys.)