Go for 6.5 or lower.
Liam says that a cause for Taiwan Bee deaths is long gestation periods. There was something else too, but I forgot. Anyway, I would get some Mosura Gravidas because I tried it because it claimed to help berried shrimp not drop their eggs, and it actually worked! Now I have 4 berried CRS (out of 9) and there were two OEBT females that berried, but they never did before.