KH/PH/GH + add rocks

[marchmaxima]

Hi all

In my 2ft planted tank, I had planned to add some rocks that I picked up at an LFS. They were marked as "river rocks" and all were very rounded and smooth, indicating to me that they had ibeen living in a stream for goodness knows how long.

I have really really soft acidic water. Out of the tap the KH and GH are 0 degrees and the PH is 7. After leaving the water sit for 24 hours, the above water params don't change.

In my tank I have driftwood, and nutrient-rich substrate which probablt are the items that are lowing he PH. The PH in my tank is usually 6.2/6.4. Typically it is 6.0 before I do a PWC. Or it might be a little lower. It's as low as I can measure anyway.

Because I am aware of how something such as new rocks can change your water (especially water with no bufferring capability, I sat the rocks in some tap water overnight before testing the PH. I'm really glad I did because the PH plummeted. so presently, I feel it would be unwise to put these rocks in my tank.

From my understanding of the issues, I undertand that I should probably focus on raising the KH/GH of my tank and give myself some buffering capability which would mean less wild PH swings if I add these rocks.

Would it be wise/unwise to add some Limestone to my aquarium water to slowly bring up the KH in the tank? Adding an airstone is not going to do anything, as I already have good water surface agitation going. It'd be nice to use these stones if I can.

 

[Purrbox]

In general rocks are going to raise your KH/pH not drop it. I don't think that the pH actually dropped because of the rocks, but rather because it came in contact with something that gave it a pH to match. When water like RO water without any buffering comes in contact with something that has a specific pH, it will match that pH. I believe your water is acting the same way.

Did the pH drop lower that it does in your aquariums? If not, you should be fine adding the rocks.

As far as the Limestone goes, I'd go ahead a pick up a piece that you like, place it in a bucket to test for a few days and see where the pH ends up. If you like the results then you could go ahead and use it. I believe with most rocks that buffer water, they tend to cause it to take on a specific pH as opposed to coninuously raising it in direct proportion to the amount of rock. That too may need some testing though.

 

[jsoong]

If you have limestone (or cc or any other carbonate source) with enough surface area, it will raise the KH & pH to its equilibrium point. For carbonates this would be pH 7.8 to 8. It does not matter how much you add as only so much carbonate can dissolve into water. <Note that it takes a few days to achieve equilibrium, so soaking overnight might not give you a true end pH.>

This property is nice in that once you add the limestone, it will act as a reservoir and hold a steady pH without any work on your part. OTOH, its equilibrium pH might not be what you want. If you want to end up at a specific pH, you would have to use bicarbonate & add a specific amount (or use a different buffer).

BTW, it is interesting that your river rock drops the pH. I wonder what its composition is .... perhaps rich in potash? River rocks sold in our area (from western Canada/US) are carbonaceous, & act just like limestone in an aquarium.

 

[newfound77951]

I would agree with Joy, I can't think of any rock that would lower pH. Water with such low buffering capacity will be hard to read for pH anyway, because there is just not much to measure.

 

[marchmaxima]

Did the pH drop lower that it does in your aquariums? If not, you should be fine adding the rocks.

I just re-tested the water where the rocks are (I haven't moved them) and the PH test kit reads 6.0. Actually, it might be lower. The colour of the water for a 6.0 reading is supposed tobe a light yellow and while this is true, it also has an orange tinge to it.

The only other thing that the water has come in contact with is the plastic container golding the water/rocks. I should also mention I have toed on some javamoss to some of these rocks (would that matter?) I have two buckets I use for fish use. I can usethese to re-test to eliminate the possibility that the plastic container is affecting the PH of the water.

If i set up the two buckets, one with water/rocks and one with just water and test for PH/KH/GH changes, I might be able to tell if the rocks are the cause or not.

 

[marchmaxima]

it is interesting that your river rock drops the pH. I wonder what its composition is .... perhaps rich in potash? River rocks sold in our area (from western Canada/US) are carbonaceous, & act just like limestone in an aquarium.

You've got me curious enough to want to look around and find out. Will let you know if I do.

I just tested the PH/KH/GH of the water in the container with the rocks, and there was no change in GH/KH - they are both at 1 degree. PH was as low as the kit could measure.

 

[marchmaxima]

I did another round of tests. This time, I had two identical buckets. Both were bought at the same time and were made of same plastic.

I setup one bucket with tap water + Prime and one had tap water + Prime + suspect rocks. Upoin setting up the buckets, the samples both tested as follows:

Upon setup: PH = 7, KH and GH were both 1d.

After 24 hours here are the results from both buckets:

Bucket of water with rocks:
PH = 6.4, GH = 1d, KH = 1d

Bucket of water without rocks:
PH = 6.8, GH = 3d, KH = 1d

Tap water
PH = 7, GH = 2d, KH = 1d

So from this I would think that these rocks don't buffer the GH or KH. Furthermore they look to definitely decreasing the PH. There is nothing else coming in contact with the water except the plastic from the bucket and IF this was the culprit, both water samples should be affected.

Asf or the geology of the rocks, I know three white marble rocks. The rest I am still trying to id.

 

[jsoong]

It certainly looks like the rocks are leaching something that is dropping the pH. I was curious, so I did some searches on rock composition to see if there is any likely candidate and came up empty. Marble is mostly CaCO3 so should act like limestone & increase the KH. My initial guess of Potash (K2O) which is common in sandstone is off the mark .... since K2O's equilibrium pH is over 8 (not in the 6's) .... so the plot thickens.

One other possibility comes to mind - organic contaminants. Decomposing leaves, etc forms acid .... if the rocks are not clean these traces of organic acids might be dropping the pH. Perhaps you can try a batch of scrubbed rocks & see if it makes any difference .....

 

[marchmaxima]

I scrubbed each rock with a brush and then boiled them for 20 minutes, just to ensure that I got as much of the dirt as I could, because they were quite grubby when I got them. There are a couple of crevices in some of the rocks that still have a bit of dirt, but no organic matter, as far as I can tell.

The batch of rocks has several types. I'll perform some more tests and see if I can determine if a particular type (or individual rock) is the culprit.

 

[marchmaxima]

I set up 5 glasses with tap water and one rock from each type. It took several days, but I tested the PH of each sample after 24 hours. Only one rock resulted in a significant change to the PH. It dropped from 7.0 to 6.2. Other than that, all the other rocks appear not to cause a drop in PH.

So the rocks (all but the one) have gone into the tank and already. I can't really tell if the PH has dropped because the PH was at 6.0 which is as low as I can test. I also did a 40% PWC and after that the PH is still 6.0. Will keep an eye on GH/KH and see if they change over the coming week.