Help me stop killing my betta fishes!

I need help.

I can actually keep my houseplants alive. I've always been able to keep our cats alive. I've even managed to keep my roses alive. But for the life of me, so far every single betta fish I've tried has died. Partly because several of them (4 out of 7--yikes!) were, as it turned out, sick to begin with. I won't buy them from Petco ever again. The occasional dead fish lying in their small plastic bowls should have been a giveaway. But two of them really did seem healthy. I've got the 7th one in a bowl now, and it's doing ok. Not great, but no cloudy around his head, either.

This time, I'm using bottled spring water, and slowly got him acclimated to it by pouring some into his bag every hour or so, until I'd done it about 5 or 6 times. Now he's in his bowl as of last evening, and this morning he's looking a little sluggish. I haven't added any chemicals yet--although I have water conditioner, Bettafix, and Stress Coat--because I was wondering if it might've been the chemicals that did it to my healthy bettas. I've been feeding him Betta Bites and he does eat, although today he waited until they fell to the bottom rather than swimming up for them.

Can anyone please help me keep this betta alive? What am I doing wrong? Should I add some aquarium salt to the bowl?

Thanks,
Kelly