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

[GreenSpottedPuffer]

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.