PH dropping after water change

Beginner here with a newly cycled 29g planted tank, sand substrate, 4 guppies.

We've been experiencing a pH drop after water changes. Our tap water pH is 7.2. Earlier during the fishless cycling process, our pH appeared to crash to 6 after adding driftwood to the tank, and we added baking soda to bring it back up. We also added a bag of Purigen to the filter to clear the tannins. (Later, I learned that it's common for pH to fluctuate during cycling, and also we were having some issues with test tubes not being adequately rinsed out, which may have resulted in that very low pH reading? Not sure.)

Our tank finished cycling last week, with parameters of pH 7.2, ammonia 0 ppm, nitrites 0 ppm, and nitrates 5 pmm per the API Master Test Kit.

We added guppies on 4/15.

On 4/16, we tested the water and nitrates had risen to 20-40 ppm. pH was still 7.2 at that point.

On 4/19, our first water change, nitrates were at ~80 ppm. We vacuumed and agitated the sand—probably too much, as there were a lot of bubbles being released—wiped down the sides of the tank, and did a 20% water change, treating with a scant capful of Prime. We have not touched the filter media. After the water change, my partner noted that the ammonia had increased to 0.25-0.50 ppm (unfortunately did not record the exact numbers), possibly due to disturbing the substrate too much. She added more Prime.

On 4/20, nitrates were down to ~40 ppm, ammonia and nitrite 0 ppm, but pH had also decreased to 6.8. We did a 30% water change with Prime and light vacuuming.

Today (4/21), I think the pH is still 6.8 (though it looked 6.6 last night, 2 hrs post WC...), ammonia and nitrites 0 ppm, and nitrates are possibly 20 ppm? Any help reading the colors would be much appreciated; I have a really hard time with those orange/reds on the API sheet. (It looks more orange when held up to the light, more red when held up against the sheet.)

My questions are: What could be causing the drop in pH? If our tap water is 7.2, shouldn't the water changes result in a more stable pH over time, rather than it lowering so much? Or is the instability of the water the issue here? Could adding another piece of mopani driftwood potentially cause the pH to drop more? We've been holding off on adding the final piece because of pH concerns.

We do not have a gH/kH test kit, but have ordered one that is arriving Thursday and will consider adding crushed coral to the filter if needed. I am also adding a sponge to the filter today, as we're currently just using the cartridge that came with our Aqueon QuietFlow 20, and I know that's not adequate for our eventual stocking plan (2 small schools of guppies and corydoras, honey gourami, and nerite snails).

Thanks in advance!

 

I am a novice just learning. You will get comments from others more experienced than I am.

I have driftwood in four of my tanks. I have not found it to impact PH at all. My KH is currently 4 or 5. Historically it has been closer to 7 or 8. I believe my KH is the reason my PH is stable. Actually higher than I would prefer - 8.2.

It seems your are on the right track with crushed coral. Odd that water changes would cause PH to drop. Usually I would expect water changes to stabilize PH.

I am not consistent with water changes. I change 20% to 50% every week or two. I have gone as long as four weeks between water changes.

I like to keep my nitrates below 10 ppm. But many say below 40 ppm is fine.

Difficult to read colors on pictures. Instructions say to read against a white back ground in bright light. I sometimes get different colors when I hold up to natural light vs bright light against a white background. I go with the color in bright light agains a white background.

 

I think your KH is likely quite low. When KH is low, tannic acid will neutralize the basic carbonates in the water, removing the buffering capacity. Tannins are not very strong, but if there's not much KH to begin with then the little effect they have will be bigger. I hope that makes sense
Oyster shell/eggshell/crushed coral or anything made of calcium carbonate will bring your KH and pH up.
In my eyes you're reading the test kits right, but the difference between having 6.6 and 6.8 pH, 20 and 40 ppm nitrate isn't too significant. Could be 30 ppm, maybe.

 

Crushed coral good. Argonite another route. Argonite is sand from the Bahamas. Crushed coral Southeast Florida. I use both for cichlids tank. Keep my ph at about 8.

 

I would not be concerned with a ph drop of 7.2-6.8 over any time frame other than something immediate. I also would not add any chemicals to buffer or increase the PH. Get the KH test kit and see where that’s at and go from there. If KH is low I’d increase KH to stabilize ph. Monitor ph and see where is goes. Do smaller water changes for the time being until you know the fluctuations of ph. Test ph of tank before and after water change to see the delta.

 

Thank you! We got the KH/GH test today and, sure enough, both are very low.
Tap water is 2 dKH and 2 dGH. In my tank, it is 1 dKH and 2 dGH. I guess there's my answer for why our pH has been fluctuating so much!

Other parameters today seem stable, thankfully. 6.8 pH, 0 ammonia and nitrite, 20ish nitrates.

I'm planning on buying some crushed coral for my filter. Is there any way to predict how much this might increase the pH? And how much coral would you recommend adding, or does it make a difference? This is a planted community tank with driftwood, and my main goal is that the pH stabilize back to around tap level. (A little higher is also fine, but I don't want it to go too high, you know?)

 

I’d add about a cup of crushed corals to each. I don’t think it will bring you ph up very much. Of it does the ph will rise slowly and not cause you any trouble. Monitor both KH and ph daily for a week or so until it settles out.

 

What substrate do you have in there? I didn't see it mentioned so sorry if I missed that.

 

Fisheye Sand substrate. I'm blanking on the brand, but it's a fairly fine aquarium sand. We're going to get corydoras, so I wanted something safe for them.

 

Thanks. Was just trying to rule out a substrate that would lower PH. Thanks for responding. And yes, Cories on sand!!! The best!

 

Crushed coral is a good idea. Also look into getting a GH booster, at 2 GH your plants are likely to develop calcium and magnesium deficiency.

 

-Mak- Thanks, I will look into that. Are GH boosters something that you can add to your filter, or would it need to be dosed on a regular basis?

 

How soon after a water change are you doing your tests?

Its not uncommon for your PH to change after a water change. It can take up to a day or more for water parameters to stabilize depending on size of tank and volume of water changed.

I'm not sure how hard your tap water is, so i wouldn't want to recommend a product for you.

Here is a link to some good buffers, I have used. I am sure one will be right for you.

Seachem - Freshwater

 

-Mak- Thanks, I will look into that. Are GH boosters something that you can add to your filter, or would it need to be dosed on a regular basis?

They're very soluble compared to crushed coral, so unfortunately they can't be left in water. You would dose at every water change. You can mix it into new water or directly drop powder into the tank. Seachem Equilibrium is a good GH booster, or if you're ordering fertilizer, cheaper at Nilocg's ferts.