Three of my tanks are suffering a PH crash, the PH hits the lowest on the API test kit, and I'm guessing it might even be lower. The problem was identified by one guppy in one of the tanks hovering at the bottom in a corner, A black mystery snails shell going hazy and Oto cats refusing to eat fresh food. Immediately seeing this, action was taken (immediate water changes, testing so on) and the PH fallout was detected.

History on all three tanks: WELL maintained. Regular maintenance performed in OCD like fashion, (lol not so OCD I destroy BB), these are not neglected tanks. Every Wednesday like clockwork, gravel vacuum (every other week is a general light sweep, the other decorations are moved and all the gravel gets a good hoover), filter media rinsed in removed tank water. Prime water conditioner used, and each tank has more than one filter. Filters are as follows:

20 Gallon - AC 30, AC 20, Elite sponge air driven filter
10 Gallon - AC 20, Cylindrical tube air driven sponge filter
5 Gallon Edge - Duetto 50, Cylindrical tube air driven sponge filter

Tank API Freshwater Test readings:

20 Gallon - Ph 6 or below (test tube color turns a weird yellow not the one on the chart, this is the same for all three tanks), Ammonia 0, Nitrite 0, Nitrate 10ppm

The nitrate on original testing when problem was noted was a skyrocket high 80PPM, it could have even been higher. This is mind boggling as the tank is pristine and well maintained. Brought down to 10ppm by water changes over three days.

10 Gallon - Ph6, Ammonia .25 (also perplexing, this tank was well established and cycled), Nitrite 0, Nitrate 5ppm

5 Gallon - Ph 6, Ammonia 0, Nitrite 0, Nitrate 0 (the nitrate at 0 baffles me as well, and am concerned cycles stalled out with low Ph)

I have done GH/KH testing, API kit, all three tanks read exactly the same:

KH (4 drops) 71.6 GH (6 drops) 107.4

All three tanks have crushed coral in the filters, quite a bit of it too, several teaspoons, and the Ph is this low. I am stymied.

Since we just had a blizzard here, I did test the water out of the tap, the PH is 7, I have not done GH/KH testing on it. It is possible however, that before this test (today) it may have been different due to the storm? This I am not certain of.

The outrageous Nitrate reading on the 20 blew me away. There is no reason for it, no detritus anywhere, filter boxes are clean, nothing dead, no rotting food, debris, nothing. The last test I did the week before just before maintenance was 20ppm. Ammonia showing up on the 10 is alarming to me, again same thing, nothing lingering anywhere, it makes no sense.

Any theories appreciated, I'm completely baffled.