This one's for my fiance. His 10g at work is totally cycled, his ammonia and nitrites are about 0, it's not overcrowded, and it gets regular gravel cleaning/water changes. All seems well until he tests the pH, which drops all the way down to 4.5! Even w/ a water change, he can only get it to 5.5 or 6.0. The water is pretty hard, so I don't think it's a buffering capacity issue, but he's going to try crushed shells in the filter anyway. Can anyone tell me what might be causing this?

That pH is extremely low! What is the pH right out of the tap? The water might be hard, but it is poorly buffered. What is the KH level in the tank?

Just a warning not to take a low ph/low kh problem lightly....It can cause major problems in a tank! Dr. Johnson from Koivet.com lost ALL of his koi due to a ph drop, and we are talking 1000s of $ here, not even mentioning the emotional attachment and value of his fish...
I had a ph scare myself last week, when my husband called me frantically to tell me that our koi ponds had VERY low ph, around 5.5..Our ponds are still 3 hours away from me, they are the last ones to move...
When he tested the ammonia, it was in the red (toxic) area!!! And the filters had been cycled for almost a year now! Thank goodness only one fish died, but he had to do a major water change, and we are talking a 1200 gl pond and 2 600 gl ponds here, and had to raise the ph fast, but carefully, since higher ph means more toxic ammonia. Its under control now, but he has to keep an eye on it, and put crushed coral into the filter or pond too, to raise the kh.
Part of your good bacteria gets starved when the ph is low, hence the ammonia problem in conjunction with a ph drop. After talking to Ingrid from the puregold list, she added that the bacteria will pop right back up after the ph is corrected, so I gave it a few days with no feeding, and its fine now. Ammonia is gone!
A cheap solution for your low ph problem would be baking soda, although I had the experience that its only a short term solution, and you have to add it every time you do water changes. I decided to add crushed coral to all my tanks inside, thats something you hardly ever have to replace, and you can have it in a cheap piece of pantyhose or something. You can get that in 10, 15 and 25 lb bags, I believe, and it lasts forever...
Good luck, and let us know on what you decide to add, and how satisfied you are with it...

I'm not sure of the kH, but I know it's reasonably high in his area... lots of calcium. We just cannot figure out what's causing the prob. Tom corrected the pH this eve. using baking soda, but he overcorrected and shot it up to 7.5. Of course, that's normally not too high, but coming from 4.5, that has to be a shock. Oooo, I should tell him to add some stress coat stuff, huh? doggone it. should have thought of that yesterday. I knew that, too.

Because we know that baking soda only alleviates symptoms without being a permanent solution, we bought crushed shells and put them in a stocking inside the filter (great minds think alike!). We'd still like to figure out what the root of the prob is, but perhaps we'll never know. Here's hoping the fish (which are at his office dealing w/ a 3.0 pH change as I type) make it through the night. If they do, what else can we do to ease the transition?

What kind of ornaments are in that tank (rocks, wood, etc)? I suggest you do a simple experiment... fill up a bottle of water from wherever you are getting water for that tank, measure its pH right after filling it up and every day after that. It will give you an idea if it is the source or something else is happening. My guess is that although the water is hard in your area, the water you put in the tank is actually very soft... softened water (since you mentioned that the tank is in an office).

I know he doesn't have driftwood in there, and he has store-bought coated gravel (better quality). I believe he only has non-reactive rocks in there; mainly lava rock (this is all based on second-hand info since I cannot visit his office... classified stuff).

Turns out the water at the lab is a little on the soft side after all. However, the fish survived the HUGE pH change (he says they seem "itchy," which concerns me) and the water is begining to stablize at near 7.5 after the addition of crushed shells.

I know they add ammonia to the water in his community, which, of course, results in chloramines. If the chloramines aren't fully removed, can they have an impact on pH as well?

Chloramines will oxidize organic matter and lower the pH in the process... but the dechlorinator inactivates the chloramines. The bacteria will also break down organic matter and lower pH... which often occurs when overfeeding or overstocking.

I suspect it's overstocked. Thanks for all your help, Kingyo!

Whats in the tank>?

actually, it's all tropicals (posted here cause it was a general tank question). He's got 2 clown loaches, 2 otos, some rasboras, a dwarf gourami (or did that die?) and is planning to add a paradise fish. I think that's what's in there. Hard to keep up since I've never seen the tank.

I think Kingyo is right. I would test the KH. Even thoug your water (GH ) is hard if the KH is low it will throw the Ph out of wack. If the Kh is low then I would go with some Coral

we bought crushed shells and put that in the filter... pH is a little too high now, but we're just going to take out some of the shells out and see what happens. thanks soooo much!

Whats wht Ph now?

It's 8.0... an enormous jump from 4.5! He's sure got some hardy fish...

WOW Dude......how are they acting ?

they're doing really well, actually! I am so amazed. Tom and I joke that the "boys at the lab" (they do all kinds of crazy experiments there) are sneaking into his office at night and running secret tests on his fish. We can't think of any other explaination...

he's down to 7.5 now after pulling some of the shells from the filter.