HELP!!!

Hey, HI, new here, and need some help. I have a 30 gallon brackish tank with GSP. Everything was boppin' along just fine until the PH suddenly DROPPED OUT. I don't know why, I do religious water changes, hadn't added anything new, etc. I LOVE my GSP, and am freaking out. Nothing I have tried has worked. I have a crushed coral substrate, I put crushed coral in a media bag in the filter, I've tried Kent marine Ph uppers, nothing is working, and my GSP is getting sick, and I'm beside myself. Any suggestions would help, I have tried a water change, and that did nothing, PH dropped right back down. I should also mention that I have several other tanks, a 55 freshwater, a goldfish tank, and a 10 gallon with some breeder guppies. The PH in all of my other tanks is pretty perfect, so I don't THINK its my water. I use Oceanic sea salt mix for my brackish tank, cud that be it? Gahh...:help: please.

The PH just dropped, like dangerously low, from the 7's to the 5's. I have a monitor that tells you the PH level all the time. Its really weird. I am not an expert with all of this, we got a crash course in the pufferness several months back, but after the initial shock...like I said, all was well.

First off, thank you thank you for the help...I do not do water changes with freshwater, I make brackish water to do the water changes, checking the SG of course. My GSP is about 2 1/2 inches, so I keep my SG and 1.014-18 (I think thats right, I didn't add extra digits.) The tank is 30 Gallons, and we have the occasional problem with ammonia, but for the most part, all the other levels are stable. I don't think the cartridges in the card need be changed, they are fairly new, but I will get the stuff to do a manual test. ( I've had horrible visions of my six year old conducting chemistry experiments with them. ) What other info do you guys need, as I am new to the forum thing, and feel like a (hehe) fish out of water...(okay, I know, smack me.) Again, thank you thank you thank you!!!

Cat