Realllllly low pH in QT tank...Can't figure this out
 

I am having the hardest time figuring this out. It makes no sense to me. My QT tank PH is reading in the low 6's. Anywhere from 6-6.5 or so. I have calibrated my probe twice this week thinking there is no way thats right. Way too acidic. My alk was tested twice tonight and both times was 3. I have no idea how this is possible. The tank is beginning hyposalinity but this started before I started lowering the SG. I am using tap water...maybe this is causing some of it but I have used tap water before and never had these problems.

The tank is 24G, has a sponge filter, air stone, lots of water movement and a bit of light from a CF bulb. Oh and two fish that seem unaffected by it all.

I am very confused and a bit worried about my fish.

I should mention that if I try raising the PH slowly with a buffer, it is back to low 6's within an hour. So no more of that because its way too much fluctuation.

That's very odd Justin, you'd think the salt would be enough to raise and stabilize the pH at 8 ish.

Yeah thats exactly what I thought. I have never had PH go below 8, so its new to me and I really have no idea how it could go so low. Im worried about the fish even though they both seem fine.

I may try AGAIN to calibrate the monitor or maybe its broken. But I did test manually as well and get the same results. This morning its reading 5.91 and alk is 30 mg/l

I thought this was way too low for saltwater fish and that by this point they would not be looking too good.

I was thinking of using a filter bag full of something like crushed coral to see if that stabilizes the PH at all. Is this a bad idea for any reason?

It should be worth a shot. At that pH, the CC should be dissolved and help.

Yeah exactly. I have CC bags in my brackish tank to keep the PH up and it works very well. I think I should do this tonight because Im getting worried about my fish in such low PH.

That still might not solve the problem. The CC should bring the ph up, but I would bet that as soon as the cc bag is removed, the ph will likely crash again, which means you haven't really solved the dilemma, just hidden it beneath the surface. Sounds to me like some kind of buffer is bringing it down.

Well since its a Qt tank that will only be up for another 2-3 weeks, I don't really need to solve the problem long term. If this were my display tank, I would be freaking out right now and not looking for a quick fix like CC.

What's your ammonia, nitrites, nitrates at?
Are you doing frequent (emphasis on the frequent) partial water changes?

0,0 and 15. No frequent changes, just once a week. This is a cycled tank, not just set up last minute for new fish. Whats the emphasis on frequent for and what does it have to do with the PH?