PH Low: Bad feeling bad things will happen

Question
I have several fish, mostly catfish. These were in a 500 gallon aquarium. They are:

Plecostomus: 10 inches
Pictus Catfish: 5 inches
Pictus Catfish: 8 inches
Pangasius catfish: 15 inches (also known as iridecent shark
x2 rainbow sharks (each about 4 inches)
Blue gourami
Kissing Gourami
x1 golden claw crab
???? ghost shrimp (buy in bulk, because other fish eat these like candy)

anyways, my 500 gallon tank started to leak, so i put all my fish in a well established 20 gallon tank (that only had a lot of ghost shrimp) PH was about 7, ammonia was less than .25. anyways, after i moved them,i had to drain the 500 gallon, repair the silicone, and refil it. I am very sure the bio filter in tank is gone, so it suffers from new tank syndrome. I refilled it, and have a few feeder reds swimming around in it. The tank still has another month until its set again. Anyways, i am having Issues with the PH in the 20 gallon with all the fish. I can keep the PH stable for a while, but it shoots downward to a number that my chart can't read (PH test gives it a Neon orange color). In order to raise it back up, i need to add a massive amount of PH higher. I also put in PH 7.0 tablets, but they do nothing. I also bought this expensive PH stablizer, but that doesn't help. After i get the PH up, it only stays up for a few days, before blasting back down. and it goes down very fast; less than 30 minutes. My pictus catfish and plecostomus have almost died from it. most of my ghost shrimp have died from it. The amonia level stays around a .5 now.

I fear if I keep them in the tank while the other one becomes established, the PH drops are going to kill everything. i there something I can do to fix the PH problem? Also, is there anything I can do to make my 500 gallon tank establish faster? My fish are very stressed from the lack of room.

Answer
Hi Chris
Wow....well, hopefully it didn't leak too bad on you before you caught it.  I've always been lucky(knocking on wood) and noticed the 2 tanks I've had leak before they got bad, and they were much smaller then 500 gallons....

Ok...I suggest to immediately get them back into their larger tank.  One good thing, with the ph under 7.0, ammonia is converted to ammonium-a non toxic form, so it's not affecting the fish.  But yes, the ph swings are and as you know can kill them.  Plus, you're nitrites will start showing up soon, and from the stress of the ph swings, they may not tolerate those too well.  True, it'll most likely still occur in the larger tank, but at least there's more water volume and space for all those fish.  

Normally I would recommend partial daily water changes, but with 500 gallons...that's kind of out of the question.  One option, and again, with 500 gallons not sure how feasible this is, but you could use Prime by Seachem.  It neutralizes ammonia and nitrites to a non toxic form without removing them, so the tank can still cycle and the fish aren't harmed by the toxins.  One large bottle treats 2500 gallons, a little goes a long way with it, but a large bottle is about 10.00.  It dissipates after 24 hours, so it'll need to be added daily.....

Or, if you know someone that has an established disease free tank, get some of their filter media and put it in your filter-as much as you can, or at least some filter "squeezings" and just dump that directly into your filter.  Now, because of your tank size, it'll most likely still go through a bit of a cycle, but shouldn't be as bad on the fish.  I'd go this route if at all possible.

In the future if you ever have a situation like this, now...also without knowing what type of filter(s) you have on the large tank, you could just let it run on the smaller tank, or remove some of the filter media and put it in your smaller tank to keep the bacteria alive.  May be overkill, but it would work.

Hope that helps and good luck!