Ph dropping A LOT!

This is my second run. First one came out decent. Could definitely improve, but was very happy with the first time results. I just started my second crop and I'm having Ph problems. I haven't changed anything as far as the setup goes. I have a 55 gallon resevoir, and in less than 24 hrs the Ph drops from 6.5 to 3.0. Obvioulsy there is something wrong. Please, don't anybody tell me to use Ph up. That won't fix whatever is causing the Ph to drop. I dumped a full bottle of H2O2 into the res, but that didn't help. What's especially strange is that the last harvest I was constantly adding Ph down. Never once needed Ph up. The plants are still in the seedling stage so I can't imagine the problem is root rot. They don't even have enough roots to make 55 gallons of water to drop from 6.5 to 3.0 after 12 hrs. I'm thinking it's some kind of bacteria, but how do I get rid of it? Can someone try to steer me in the right direction before I lose it and take a bat to my entire grow room.

dump it and start again...clean it real good..i bet there is shit in the magnetic pumps still. they do attract foods being a magnetic pump...if it is one. only way other than that to drop ph is someone is adding more food or they arenht taking any in and you keep toppin it off???

what is the ec/ppm

I haven't begun using any nutes yet. Just tap water, which is 300ppm. I took the seedlings out just now and dumped a half gallon of bleach into the 55 gallon resevoir. There is no way anything is surviving that. So if it is bacteria, that should fix it.........hopefully. I just hope I flush it good enough before the plants go back in.

ok now also take a cup of water and ph it and see if a cup does this., also check ph pen batteries, thats usualy first thing i look at iof shit is fiunky like this. make sure clean that bleach out good after. if its a mag pump it attracts the iron in the water..but in theory that should rasie ph being less of an ec....so ya check your water but not in that same set up. you may have shit in the water itslef doin this.