A few last things,
Do you change out your filter cartridges?
Do you still try to use pH changing additives?
1. Acclimation - The single thing that sticks out to me most is the acclimation of fish. The temperature matching method just isn't enough. Check here for a run down of proper acclimation procedure
Acclimation Procedure for Aquatic Life: Welcoming Home New Arrivals
Not acclimating fish properly can cause osmotic shock and severe damage to the fish which would include possibly killing them.
2. Water testing - The test strips are basically junk. They aren't particularly precise and they don't have an ammonia test which is very important. I would highly recommend investing in this
http://www.amazon.com/API-Freshwater-Master-Test-Kit/dp/B000255NCI
They last a long long time. One kit should last up to around a year depending on how often you test.