Puzzling parameters in two tanks

[Kunsthure]

I've been keeping track of params in my 13g almost daily. The KH has stayed constant at 2, GH was back and forth between 4-5. pH was at 7.4 for the first three days after a WC, then dropped to 7.2 for two days, then to 7.0 for two days. It's been five days since I've tested and today, WC day, it's 6.8. How is my pH dropping when my KH and GH are the same? There is a new piece of DW in the tank, but I thought my KH and GH would buffer that.

Second, in my 20g, the params are usually GH 4, KH 0-1 and pH 6.6. Since I moved my RCS to that tank, I added three coral rocks to it to raise the GH, KH and pH for them. So I tested today, checking if the rocks made any difference. The GH is now 8 (my tap is 6), KH is still 0-1 and pH is 6.8. I thought the coral rock would raise the KH but not the GH, and have a greater impact on the pH.

What's going on?

-Lisa

[Diana]

Tank #1:
GH varying around 4-5 German degrees of hardness: Could be a test kit problem, you might be adding Ca or Mg with some fertilizer or other additive, then the plants are using some of these minerals or any of several other things. Don't worry about it. I would still call that stable.
KH 2 degrees is so low the pH can easily change.
pH slowly dropping. Could be the driftwood, or it could be other things going on in the tank such as organic matter getting broken down. The KH is so low it is not stabilizing the pH. This is the least of the 3, so I would ignore it. Blame it on the driftwood.

Tank #2:
GH rising with the addition of coral or rock might be because the coral or the rock is more soluble in some ways than others, or else something in the tank is using the carbonates as fast as they are becoming available. Nitrifying bacteria use carbonates as a source of carbon. Some plants can use carbonates as a source of carbon, too. Some substrates remove KH, but leave the GH in the water.

[Kunsthure]

Tank #1 has to have a GH of 4-5, KH or 2 and pH of 7-7.4 because it's going to be an OEBT tank. I can't have pH swings like that. Should I do two small WC per week to get rid of tannins? I am going to run Purigen, will that take care of the tannins? This one is irritating me because I thought I had everything set for the shrlmp.

I've had PFS in tank #2 for around two years. The params are so low in that tank because of the airstones (tap is pH 7.8, GH 6 and KH 4). I have PFS in my 37g and params match tap. There have been plants in breeders nets, wendelovs and crypts while I've been trying to sell them, but not many. Would they really make that much of a difference?

-Lisa

[Diana]

Tank #1: If you are dead set that you must have a higher pH than the KH will buffer it to, but do not want higher KH then you will have to use some other chemical to raise and stabilize the pH. Purigen will remove tannins.
Is this a planted tank? Are you measuring the pH at the same time each day? pH will rise through the day as the plants remove the CO2, then drop at night as the water takes in more CO2. Maybe your earlier (higher) pH readings were tests done in the later afternoon, and were testing CO2-depleted water. Then the lower pH tests were done in the early morning before the plants had a chance to remove much of the CO2.

Tank #2: IME air stones do not alter GH, KH or pH beyond the first 24-48 hours. IF the tap water has a CO2 level different from what is natural for water when it is in equilibrium with the air, then aerating the water will help it reach that equilibrium faster. This will not have any measurable affect on the GH or KH. All it is doing is circulating the water so the water is in better contact with the air. Sounds like too small a plant population, and slow growing plants, too. I do not think that is enough plant growth to noticeably affect the GH or KH. I do not think Crypts or Ferns can utilize carbonates. (I might be wrong about about that), and while they do use calcium and magnesium (GH) they grow so slowly that I would not think you could measure a drop in GH.
From what you describe I do not know why the parameters in tank #2 are so different from the tap water.
PFS: Not chemically active.
Small, slow growing plants: Not enough to do this. (Drop the KH so much)
Coral or rock: Ought to add minerals (and yes, GH is rising) and should have added KH and raised the pH. But it is not. Perhaps get some coral sand or limestone sand and add that to the filter in a bag (nylon stocking is a good material for a bag). Use something different from the coral or rock you are now using. The greater surface area of sand will allow the reaction to happen faster. Greater contact with the water. Or, you could add baking soda to raise the KH. Raising the KH will usually raise the pH. Doing more frequent or larger water changes will also raise the KH because you are adding KH from the tap water. Vacuum the sand, too. Remove any decomposing matter. Decomposing material will remove KH and drop the pH.

[Kunsthure]

Tank #1 is all moss now and pathetic frogbit/salvinia the mysteries keep eating. I have been ferting it daily while only snails are in there to encourage more moss growth awaiting my shrimp next month. I had been testing at 6-7pm until yesterday when I tested around 4pm right before the WC.

As for tank #2, my tap is pH 7.8, GH 6, KH 4 and the only thing different about that tank from the others is the airstone. When I had an airstone in tank #1, it had the same lower params as tank #2. The only thing in common between those two tanks was the airstone (tank #1 had tahitian moon sand and now has flourite, so no active substrate). The 20g is my 5 y/o daughter's so it's all neon fake plants except for two pathetic water wisterias and some frogbit/salvinia; I moved the crypts and ferns to my 37g so they'd get more light while waiting to be bought. I often test the water right before the WC and that's when I get the lowered params.

I do have some crushed coral I can try. So I guess I'll add that. Will it work the same if it's not in the filter, but just sitting on the substrate? My filters are crammed full of extra biomedia and filter floss so there's not really room for a bag of crushed coral.

-Lisa

[Diana]

The more water flows through the crushed coral the better. The faster the reaction. If you cannot put it in the filter, put it wherever there is the maximum water flow. If you know the water flow from the filter hits the substrate at a certain point, then that would be a good place. Maybe hang it from the back of the tank sort of under the filter outlet?

[Kunsthure]

That sounds like a good plan, I'll have to try that.

Thank you so much for your help!

-Lisa