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Issues lowering ph

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Hey all,
I've been racking my brain for a while trying to find a solution to my problem and bugging a few other people for their opinions and have come to the conclusion of something strange going on so here I am asking for more advice on here!

Firstly some background on the problem.... This is NOT for a shrimp tank, it is for an apisto tank. I am trying to reduce the pH in one of my tanks to be between 5.5 and 6. To do this I am using RO water and drip filtering this (small handful in a cup that water drips through as it fills the drum) through peat moss which brings the water ph down to ~5.8pH , 0TDS with a kh/gh of 0.

The starting pH in the tank was 7.2-7.4pH and the starting TDS was ~150, unsure on original gh/kh but tap water was used on the tank prior and that is 3kh and 3gh so shouldnt be far off. Current paramaters are 7.0-7.2pH, 20TDS, 0-1kh and 0-1gh, i will explain below how I got the paramaters to this point.

The tank is a bare bottom breeding tank, and has nothing that should effect pH at all in it. Here is a list of things in the tank:
- 2 x sponge filters
- 5-6 x Marine pure spheres
- 2 x coconut cave (originally had 1 coconut and 1 terracotta pot but have since removed the terracotta pot for testing, explained further below).
- a bunch of oak leaves, loquat leaves and a few indian almond leaves.
- Some java moss, java fern and some anubias.

Shot of tank prior ot removing the terracotta pot:

I have done 6-7 water changes in total ranging in size from 50-75% and the ph has not moved from what it is while the TDS goes down each time by roughly the same percentage. After the last water change I decided to test the gh/kh also, which came back at 0-1 (1 drop) on the api kit for both gh and kh. last water change was 4-5 days ago now, i have since pulled out the terracotta pot and put it in a tub of RO water and it has had no effect on pH but am going to test gh/kh tonight. pH now remains at 7.0-7.2 (the last water change, of 80%, had a slight effect on pH but could be the pH pen reading slightly out now), and kh/gh is still 0-1. 

I am considering doing another water change tonight if I get time to fill the drum, again filtering RO through peat but this time using more peat (2-3 handfuls) to get the pH even lower then usual and see if doing a 25-50% water change has any effect on the pH.... I also now need to start re-mineralising the water back to ~50 TDS to bring it back up as the TDS is currently fairly low at only 20... Doing this with Salty Shrimp GH+ at the moment (plans to swap to JayC's DIY mix after this runs out).

All testing of pH and TDS is done with an electronic meter (usual cheapies from ebay) but I am confident that the issue is not with them as they measure the lower ph fine. (I've mixed water up as low as 3.8pH previously and it measures fine).

Any suggestions for what I am doing wrong or what could be causing the pH to not budge?

Thank you in advance for reading the long post and any help/advice offered :)

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I've used some driftwood and limestone. Helped me a bit. My pH was about 8! so now sitting at 7.5. Def worth trying it out? 

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For anyone who finds this thread in the future, I believe the cause was the terracotta pot. From my understanding on some info from another forum, the terracotta can react and with ion exchange it can remove acidity from water which would in turn buffer pH over time. I believe this to be the case in my situation.

I removed the pot to 1L of water and it is 80% submerged 3 days ago. Starting pH was 6.5 and 8 TDS. 24 hours later i did a water change and dropped the pH to 5.5, TDS now 5. 48 hours or so later and the pH has gone back to 6.5 and TDS has increased to 35. I will keep it in the container for a little longer to see what happens.

The tank in question has gone down from 7.2 to 6.6 since removing the pot. It is now stable at 6.6, i will be doing a water change to drop pH further this weekend. 

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It's interesting that you found it was the terracotta pot. I have some terracotta pots with plants on in my 24" tanks and have no problem with them. I have been using them for at least 2 years and my ph is consistently 5.5. Please keep an eye on it to make sure that is your problem. Mine are covered in plant on top but open inside so the terracotta is open to the water. I have them in at least 4 tanksimage.thumb.jpg.19b6177344dd3f6bcd06298d

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@ineke I am quite confident this is my cause as the pH has dropped in the apisto tank and gone up in the container. I don't think it's going to happen with ALL terracotta, i've used them in past with no issues about pH

My understand is that the ion exchange capacity of the terracotta would only be noticible in a tank that didn't have other things buffering pH up or down such as co2 or aqua soils (like your tank). It may not occur with all terracotta I guess, would depend on the manufacturing process and so on. You can destroy the capacity as outlined in the quote i'll add below.

Here is what I was told on another forum:
The terracotta pot may be acting as an ion exchanger, which it doesn't really leach anything into the water but it may 'capture' acid and remove it from the water. If there is none (or not enough) you won't see an effect on the pH It will just depend on the ion exchange capacity of the terracotta/ceramics how pronounced the effect is or how long it lasts.
To destroy any ion exchange capacity you could store in a dilute hydrochloric acid solution for a few days then in RO water for a while

My pot has been in a tank for 6+ months I recon, but in the apisto tank for a month or so. In regards to your pot, your tank will be buffered by the soil. The power of the soil will overdo the strength of the terracotta pot ion exchange. 

I'll be doing a water change tonight so i'll see what happens to the pH in the apisto tank now that I have removed the pot :)

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Glad you sourced good info. I was just worried you were putting all your trust in something one person may have mentioned. As always what works for 1 person doesn't always work for the next. Just keep your testing going a bit longer before you don't worry about it. I routinely check all parameters monthly now that my tanks are so stable unless I notice a change in shrimp activity. I'm glad you have found your problem

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Ooo I just saw this post.

Glad you have found the culprit of the pH rise.

But if I saw it earlier, I would have pointed out to you that the Terracotta pot is the suspect. There is really nothing else in your tank than can be a cause.

In fact terracotta pots are sometimes used in Cichlid tanks for that very reason, it raises pH.

While I don't dispute that it is acting as an ion exchange, as per the info you received, where it traps acid and removes it from the water, it's certainly plausible. Although, I'm personally not sure if terracotta pots can actually do that. A low pH in water is from the lack of Hydrogen molecules in H2O. 2 Hydrogen molecules to 1 oxygen molecule. When the hydrogen H molecule is removed ... pH drops. The reverse is removal of the O molecule from H2O leaving only Hydrogen H molecules ... thus making it more alkaline. Terracotta can trap minerals and molecules. But it cannot remove molecules from elements. That is, it can trap whole elements like H2O or NO3. But it cannot, for instance, remove O from H2O, leaving only Hydrogen H - causing the pH to rise, making the water more alkaline. So as you can see it cannot trap "acid". 

I offer another more plausible explanation....

Terracotta pots are made of clay. The iron oxide in the clay gives it it's distinctive red/brown colour.

Older terracotta pots were made from clay only. HOWEVER, more recent terracotta pots especially cheaper ones are now made with added lime. Only a potter would know why. Maybe it's a filler, so you use less clay. Maybe it makes it easier to mould. Maybe Lime is cheaper. I dunno. And I am not suggesting that ALL terracotta pots are made with added lime (you'd have to ask the manufacturer). 

But it is this lime that is causing your pH to not budge. Terracotta is very porous. There isn't enough lime in the pot to raise pH drastically  and since it's baked into the pot, it's not very accessible. But there is enough lime in the pot to buffer your pH in what looks like a VERY small tank (what size is that tank?).

The small tank and small volume of water is also a key impact. You would be unlikely to experience the same buffering of pH in a larger tank. In a larger tank you would be unlikely to notice a difference at all. And if you added plant substrates, like ADA, BEP , etc that buffer the pH down, you would be unlikely to notice again.


Combination of a couple of things:

- a new, cheap terracotta pot made with some added lime in it.

- small tank 

- bare bottom tank with no pH buffering substrate.



<edit> This, I believe, is why you hear of some people not noticing an impact to pH,

whilst some other people have issues with it.

Some people are using pots that have no added lime. Or some people are using it in larger tanks, and maybe with shrimp/plant substrates that help lower pH. Or a combo of all of them.

Edited by jayc

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Wow thanks for the informative post @jayc :) I understood very little about the chemistry side of it, but understood the concept haha. It is possible that this pot is made of clay and judging by the rise in pH in the container outside of the tank I would say it most likely has SOME lime in it. Pot was kept in other tanks for 6+ months without issue but they were co2 driven tanks so obviously didn't have enough strength to effect them negatively.

Tank is ~35L of water volume (46x28x30cm).

I actually found out that my issue is STILL occurring as of today. However, it seems without the terracotta pot the pH buffers to 6.8 rather than 7.2. I did a ~50% water change today, using water with a pH of 5.5. Was expected the tank to drop down to somewhere between 6 and 6.5, but alas it went back up from 6.6 to 6.8!!! Very frustrating!! 

The pot is definitely part of my problem as it has risen the pH in the container I have it in right now, so I am not going to return it to the tank.

The only thing left in the tank apart from plants and sponges is marine pure and coconut caves. At the moment I am not suspect of the coconut caves as they are wood based, and the ones in the tank are actually fairly old (1+ year in tanks) and stopped leaching tannins a long long time ago. 

I have now removed the marine pure spheres from the tank (have read about a few issues with regards to ceramics actually causing issues... so i figure why the heck not try it ) and put them in my fry tank so they don't lose bacteria. I have also put one of the spheres into a cup of RO water, will monitor this over the next few days. As well as the tank itself. If i see no change after this ill rip out the coconut caves and see what happens then, but I highly doubt this is my problem. Only thing left after this is moss, ferns or sponge filter which I would think would not have any effect on pH???

Edited by s1l3nt

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