I hate to resurrect older posts, but I was in the middle of doing Darby's 100% change today and decided to check the PH in the aged water compared to his old water. Come to find out *gasp* his old water somehow managed to be at 8.0, while the aged water was at 7.2!
I had to share the discovery in the off chance it may have been the cause. You might want to check your PH against the old tank water.
I got mine at Petco for $30 I believe. It's a "master" test kit though, so it tests ammonia, nitrate, high-range PH, PH and nitrites. But I think you can get them separately for around $7 each.