Ph crash

Bacterial activity creates acid which consumes Kh. If you were at 7.0 withut injecting Co2 your Kh was at or below 1 DKH. So when it got eaten up, the Ph started dropping. your water change might have added a little back in but probably not much if your Ph was previously that low. whatever was added will be consumed rapidly as well which will more or less maintain the yo-yo effect. I would slowly and carefully bring the Kh level at least to 2 DKH unless you have a planted tank with co2 then I would take it up to at least 3 DKH. and maintain it via water changes from there. If you find your water changes aren't adequate to keep the kh higher, add some crushed coral to the filter. You will have to play with quantities to get the right amount though.
There are plenty of threads here on just this subject so if you run a search on Kh adjustment you will gett all of the info you need.

BTW if you have peat in the filter it will cause the crashes as well when you have no KH. Driftwood can contribute, but really doesn't do so much after the first couple of weeks.
Dave

What is the pH of your tap water after it has sat out in a shallow bowl overnight?

Jim

It's hard to say. You really need to know your tap water's overnight pH for starters.

It is not uncommon for tap water treated by certain water treatment processes to have pH that drops when exposed to air. Some water treatments drive out carbon dioxide (which is acidic in solution) making the water more alkaline. As soon as the water is exposed to air, it picks up CO2 and the pH drops. That's why you need to know pH of aged tap water before you start trying to make any changes.

HTH,
Jim

That would put your Kh level below 1 DKH. I would look at ways to raise it at least slightly, to stabilize things. You should get a Kh test before you do this, but can work carefuly with only Ph if you aren't adding Co2 to the tank.
Dave

I think Dave is probably correct, and you have very little buffering (expressed as KH or carbonate hardness). Assuming that, your water is very susceptible to even small amounts of acids, which will lower the tank pH. Acids are produced in the oxidization of ammonia/nitrites, which in your tank (stocked on the heavy side) will be a factor. The driftwood may or may not be contributing substantially, but it will generally tend to lower pH as tannic acids leach out of the wood.

I'd add a tablespoon of crushed coral to a nylon stocking and drop it in the filter. You should see a small but stable increase in pH and a concomitant increase in GH. A GH of 3 degrees or 80-100 ppm will yield a relatively stable pH that isn't too much higher than what you have now.

HTH,
Jim

I have been using 1/4 inch limestone chunks in the filter and some larger ones scattered throughout the tank (maybe a pound or two). My Kh seems to be stable but I continue to have to add baking soda to the water changes to keep it around 3-4d.

my tap is the equivalent to distilled water. no gh no kh ph 7 no nitrates no phosphates. possibly a little chlorine.

if i increase the limestone that will create a positive delta and force me to do water changes. that might be a good thing. hmm