Rocks that turn water acidic

Buffering Capacity (KH, Alkalinity)

Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The ``sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.

Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.

In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness'' (KH), ``alkalinity'' and ``buffering capacity'' are used interchangeably. Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity'' should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH > 7).

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Back in the fold said:

This is one of those things that are very difficult to impossible to tell from a picture but I see lichen on one of those rocks . That shouldn’t turn your water acidic but if these rocks were not washed prior to putting them in your aquarium then they might have rodent urine on them that would . Rodents eat lichen and mark their territory .

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TwoTankAmin said:

Rocks that are soft, you can easily scratch them or break them are normally the ones which will increase pH. They do this because the water is acid and it will dissolve the rock. We can test rocks to determine if they might increase pH using some form of acid. I have Muriatic acid I use for lowering the pH in one tank, so I would use that. But it is strong stuff and not for inexperiencced users. However, vinegar is quite acid and can be used.

Put a bi if the acid/vinegar onto the rocks, If the result is fizzing, you know that rock can raise the pH. The more ecid the water is, he faster it will dissolved the rock. If we want to raise the pH in a tank we must raise th KH. In tanks this is mostly caused by carbonates and bicarbonates. So we are not limited to rocks for increasing the KH, and even the GH in some cases, we can use crushed coral, shells etc. Anything containing or made from these things will be dissolved by acid and then work to increse pH.

Making water more acid involves the reverse process. One need to lower the KH.

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Uberhoust said:

Found a brief paper on acid runoff from mine tailings here. https/uwaterloo.ca/wat-on-earth/news/acid-mine-drainage-past-presentfuture#:~:text=The generation of acid mine drainage (AMD) and,these materials are exposed to atmospheric oxygen (O2).

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StevenF said:

Dolomite is CaMg(CO3)2 There is noting in dolomite that would make water. Generally dolomite react with acid releasing CO2 and that pushes the PH up to about 7. Once the PH is above 7 the dolomite stops reacting and PH stabilizes at 7 or very close to it. The CO2 released can make the water acidic but but in my experience it is hard to get enough CO2 in a tank to drop the PH that much. You would basically need a prssurizeed CO2 system to do that. In any case after a few days in the glass the CO2 should outgas and the PH may change.

Looking at your pictures I cannot tell what those rocks are. But if you expose the rock to vinegar and it fizzes it is probably a carbonate rock of some kind. If it does not it is something else.

Mine tailing runoff is often the result of sulfide minerals reacting with water and are resoling in metal oxides and sulfuric acid which often lead he's other minerals and sulfur out of the rock. SO2 can easily reduce the PH to 6 or less. With enough a PH below 4 is possible.

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Uberhoust said:

There is still what appears to be iron staining on the rocks, and they appear sedentary but I wouldn't bet the farm on that statement. I don't think I can ID the rocks without some concept of what to expect in the area, and I don't know the basics of the general geology in Australia, other than it must be interesting given all the rocks that come from there.

Getting to your issue of the PH dropping, if it is an issue.

1. The rocks may be the cause. To test simply place them in water aged a day, and monitor the pH.
2. The rocks are not the cause. It is possible your tap water has been adjusted for pH and it is stabilizing after it comes out of the tap, the normal pH is 6 but is pushed higher as a water treatment to reduce corrosion on the community water supply.
3. The rocks are not the cause but something else is causing the drop in pH, maybe some tannic or humic acids introduced with the substrate, wood, or other additions to the tank.

A closer location would allow us to look at surface geology maps, of the area.

Just a thought is that pyrite is pretty common in some sedimentary sourced rocks. If you look at the rocks with a magnifying glass do you see any cubic shaped small crystals? Typically these should already be weathered out but maybe you have some fresh faces on the rocks that you collected. Pyrite can cause acidification.

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