I recently added some guppies to a 10 gallon tank, with the intention of using the fry as feeders. I had fry within 24 hours and learned guppies are cannibalistic. The male died the next day so I tested my water and found the PH to be at 6.4 I did a water change (40%) and retested next day and was still at 6.4 Tap water tests at 7.3, I add Prime for a dechlorinater and test again at 7.3 I used Proper PH 7.0 (my target level) and get no change. I used PH UP and get no change. I do more water changes (with the 7.3 tap water) DON"T use Proper and get no change. Take water to pet store to make sure it's not me or test kit, it tested 6.4 HELP!!!!!!!!!!! Why is it going down that far and why can't I get it to come back up. The samething is happening to my 55 gal tank with no negative affects to date. I'm not 100% sure the PH level killed the guppie but.....? FYI I've tried different amounts of Proper, half as much , too much etc. I'm boggled!