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So I've had this problem recently with ph swings. There was nothing in the tank, but for some reason, after water had been in the tank, it would drop to an extremely low ph. I'd put the tap water in, 7.0. Then 3 or 4 days later, it would be 5. Now I've done everything including throwing out gravel, decorations, but nothing has helped. I thought I had finally solved the prolem when I bought a new test kid and got a solid reading of 7.0 for 1 week. However, I was wrong. Today, right as I am about to add my first batch of fish, I got a reading of 5.0. Why is the happening and how can I fix it in time? Now, although long term solutions are good too, what i would really appreciate is some advice on what to do with the fish I just bought. I just picked up 6 platies with the goal of using them to cycle and I'm afraid that if I put them in they'll die. I have an empty (as in no bacteria, no water even) 2.5 gallon tank that I could put them in while Ii straighten this out but I'm afraid they'd die of ammonia posioning long before I get the chance. Yesterday the water was fine, but today It's suddenly dropped. Any ideas on how to adjust (if possible) the fish to a 5.0 ph?

It sounds like you have very soft water that has no way to buffer itself from disolving carbon dioxide into carbonic acid from the air. I normally wouldn't recommend adding chemicals to change pH in a tank, but you might benefit from using a little baking soda.

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What do you use to measure the softness of water?
Is that what KH and CH is for? I don't have a test kid for either.

Get a kh test done and if possible gh. Most stores that sell fish will do it for free. If it's below 3-5dkh(degrees carbonate hardness) then that's your problem. It doesn't have enough buffer to hold the ph stable. You can add baking soda or buy commercial buffering powders. I use Kent's ph stable and they also sell RO right which will add general hardness minerals(gh) along with buffering if your water is also low in gh.

I would take two good water samples to your LFS one from your tank and the other from your water supply. Do a test youre self first. Also take the test kit with you just in case there is some thing wrong with it. If you are in the same area as the LFS there water supply should be the same as yours.

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Also, to keep the hardness up at an acceptable level long-term, you may want to consider using a crushed coral substrate (or at least mixing some in with your existing substrate).

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I'm not your neighbor, you Bakersfield trash.

If the test shows you do have a low Kh use oyster shells instead of coral. I use to keep crushed coral in the extra media basket of my emperor and it would keep my ph stable between weekly water changes (tap is 7.0 ish, almost 0 Kh. One day I tested and my ph was 6.0 or under, but the crushed coral was still in my filter and it had only been a week since the last w/c. After some research I found this articlehttp://72.14.205.104/search?q=cache:QafQ7RBDC30J:www.koivet.com/handouts/ph.doc+low+kh+crushed+coral%5C&hl=en&gl=us&ct=clnk&cd=9

Main point being... "Crushed Coral is Calcium Carbonate and Magnesium Carbonate but also contains a central structural molecule of Calcium Hydroxy-appetite. (I didn't name the stuff.) The significance of this is that when the crushed coral dissolves, it leaves behind the insoluble Calcium Hydroxy-appetite which is not a contributor to pH stability. While crushed oyster shell dissolves and vanishes, letting you know when to add more, the crushed coral remains, looking good but doing nothing."

Oh and here is the effect of your low ph on the bacteria in your tank http://www.bioconlabs.com/nitribactfacts.html

"

The optimum pH range for Nitrosomonas is between 7.8-8.0.

The optimum pH range for Nitrobacter is between 7.3-7.5

Nitrobacter will grow more slowly at the high pH levels typical of marine aquaria and preferred by African Rift Lake Cichlids. Initial high nitrite concentrations may exist. At pH levels below 7.0, Nitrosomonas will grow more slowly and increases in ammonia may become evident. Nitrosomonas growth is inhibited at a pH of 6.5. All nitrification is inhibited if the pH drops to 6.0 or less. Care must be taken to monitor ammonia if the pH begins to drop close to 6.5. At this pH almost all of the ammonia present in the water will be in the mildly toxic, ionized NH3+ state."

While, Nitrobacter's preferred pH may very will be 7.3-7.5, that does not help home aquaria since recent research Nitrospira is the dominant nitrite oxidizer in home aquaria. See Hovanec et al., Applied and Environmental Microbiology, 1998. -bacter is dominant in waste-water treatment, and the older literature just assumed that that was true for aquariums too. The new research is where Bio-Spira got its name, and why it seems to be more successful than its competitors, since it has the -spira in the bottle rather than -bacter like its competitors.

All that said, too, the bacteria will grow in almost any water, and patience is needed. The bacteria for aqaria are rather slow compared to some of their fellow bacteria anyway, in that aquarium bacteria double around 24 hours, while most bacteria are far speedier than that. E. coli is around 8-10 hours, and some bacteria can replicate as quickly as 20 mins per generation. So, attempting to mess with the pH to try to get the preferred growth rate so save a hour off the average growth rate is probably not worth it since changing the pH of water can be very tricky and necessitates adding several chemicals.

Finally, the toxicity of ammonia at pH of 6.5 does not just depend on the pH, also on the temperature and concentration of total ammonia in the water. Please see these charts I just posted: http://www.fishprofiles.com/files/threads/31366.1.htm

I think if you just wanted to add something to the tank a chunk of limestone rock would be far easier. You can get it at almost any store, it will definitely raise the ph, and it will take a heck of a long time before it's broken down enough to stop working. The problem is when you add things to the tank instead of dosing a set amount of buffer you have less control. You could end up with a higher ph than you wanted or if you don't watch it and change even the oyster shell as needed your ph could drop. If you don't care what ph you end up with then limestone is your best bet since it will probably never require any maintenance to buffer the water but it may give you a slightly higher ph of around 7.8. Our tapwater runs through limestone and has a ph of 8.6 and in dry weather 9.0. It does not take me any longer to cycle a tank with high ph water than when I diluted it down for the ram breeding tank. Ph does not affect cycling time or bacteria populations a noticeable amount.