From what you said I don't get it unless just bad diseased or damaged stock to start with.
1) Your salinity is fine. Over the years I have raised them in tanks ranging from 1.022-1.026.
2) I would question the light that you refer to as just white and blue LED. Although I can't see them dying in just a couple days BTA's like a lot of light and have always been grown best under MH. Although the RBTA's I have now still grow and reproduce like weeds under Kessils.
3) I have only ever had one BTA die (that I recall) and that was recently when I sold a bunch and I damaged the foot on one removing it from the tank. Are you sure they are not damaged when you get them?
4) As far as long established tanks not true. I cycled my current tank in 2 weeks and started adding stock. The original RBTA in this tank was added sometime in the first 6 weeks and in 21 months with no effort turned in 16 RBTA's.