I mean test for ammonia etc. it's unlikely that it is the cause but it's worth knowing. I have crappy city water and do %90 water changes weekly and my little guys don't seem to mind . If you really want to get to the bottom of it you can research different water adjustments . I.e. Hardness ,etc and try to tweak them for your axies but from a little research I've done and personal exp these things matter very little . I just check ammonia nitrite , nitrate , and ph daily to ensure there health. And aqueon should be fine as long as is dosed correctly sorry if I haven't been the best help