Using limestone to increase pH

[dmaccy]

Question for all, I've been having a discussion with a mate and would like some opinions/experience.

Can you use limestone to raise pH to a specific level? I.e. can it be used to raise pH from 6 to 7 or can it only raise the pH to a specific level of equilibrium, I.e the higher pH we aim for in cuchlid/reef tanks

 

[TwoTankAmin]

The below is from the FINS site, The underlining was added by me.

Hardening Your Water (Raising GH and/or KH)​

The following measurements are approximate; use a test kit to verify you've achieved the intended results. Note that if your water is extremely soft to begin with (1 degree KH or less), you may get a drastic change in pH as the buffer is added.

To raise both GH and KH simultaneously, add calcium carbonate (CaCO3). 1/2 teaspoon per 100 liters of water will increase both the KH and GH by about 1-2 dH. Alternatively, add some sea shells, coral, limestone, marble chips, etc. to your filter.

To raise the KH without raising the GH, add sodium bicarbonate (NaHCO3), commonly known as baking soda. 1/2 teaspoon per 100 Liters raises the KH by about 1 dH. Sodium bicarbonate drives the pH towards an equilibrium value of 8.2.

Raising and Lowering pH​

One can raise or lower pH by adding chemicals. Because of buffering, however, the process is difficult to get right. Increasing or decreasing the pH (in a stable way) actually involves changing the KH. The most common approach is to add a buffer (in the previous section) whose equilibrium holds the pH at the desired value.

 

[Alice B]

Limestone will do it. Too much limestone in alkaline water will promote algae and/or cyanobacteria (BGA) growth. Once you hit 7 take it out

 

[Colin_T]

Add a small amount of limestone and monitor the pH over 2 weeks. If it's still too low, add a bit more limestone and monitor the pH over 2 weeks. Continue adding limestone and monitoring until you get the desired pH in the tank.

Check your tap water's pH. If it's low, you might want to buffer it before adding it to the tank. You can use limestone for that too. Just add a few pieces of limestone and let it sit for 24 hours then check the pH.

 

[Alice B]

however, in areas with high pH (8) and a KH of 6, that limestone can spike the pH and act like plant and algae food

 

[OnlyGenusCaps]

Limestone will do it. Too much limestone in alkaline water will promote algae and/or cyanobacteria (BGA) growth. Once you hit 7 take it out

however, in areas with high pH (8) and a KH of 6, that limestone can spike the pH and act like plant and algae food

This is a new one on me. May I ask, where you get this belief from?

 

[Alice B]

This is a new one on me. May I ask, where you get this belief from?

Servicing ponds and aquariums over a wide area of north Texas for over 20 years. Limestone isn't a problem where water is soft, pH 7.6 or below, but put it over in fort worth or grapevine and I have pictures. A lot of people do. People built ponds with native limestone. The definition of a pond built of native limestone is "algae pit". The smell of cyanobacteria is often present if the pond is in the shade instead of sun. Cyano instead of algae. 800 pound clumps of aquatic iris.

 

[seangee]

I would focus on getting the GH and KH where you want it as per @TwoTankAmin's post. This is far more important than pH. Let that do what it wants, get the other 2 right and you will have a good environment for your fish.

 

[OnlyGenusCaps]

Servicing ponds and aquariums over a wide area of north Texas for over 20 years.

Got it, personal experience. Thanks for explaining! It's interesting because my personal experience isn't similar at all. I've used loads of limestone in my hard, high pH water and it always seems benign. On top of that, Texas holey limestone is used often, or used to be, by thousands in African cichlid tanks with no ill effects. I always figured that the limestone would be chemically less reactive under those conditions and therefore not an issue. I suppose I was just surprised your experience was so different.

 

[Alice B]

Got it, personal experience. Thanks for explaining! It's interesting because my personal experience isn't similar at all. I've used loads of limestone in my hard, high pH water and it always seems benign. On top of that, Texas holey limestone is used often, or used to be, by thousands in African cichlid tanks with no ill effects. I always figured that the limestone would be chemically less reactive under those conditions and therefore not an issue. I suppose I was just surprised your experience was so different.

it may have to do with stocking levels, co2 attacking. Holey rock is pretty stable but the native limestone in the fort worth area is softer than reef rock and breaks down much more quickly, CO2 given off by fish in heavily stocked ponds or aquariums attack it quite successfully.

 

[jesusjones]

I keep limestone rocks in my filters for goldfish, I always considered it a buffer. If pH drops much lower than 7 they start dissolving. Limestone isn't dissolved by hard water. Acidifying bacteria, maybe.

 

[jesusjones]

co2 attacking

acidic conditions

 

[Alice B]

alkaline water with co2 being constantly given off. Many ornamental ponds are overstocked, While the tap water is alkaline and the pond itself is at least 7.5 if not 8, the CO2 is dissolving soft limestone, turning the calcium carbonate into plant food as well as keeping the pH up. concrete ponds have a similar problem, water is always green unless a UV is present

 

[jesusjones]

Had to bust out my notes, above 8 pH there wants to be no dissolved CO2. So you force a reaction something like this...
CO2 drives the formation of H+ and CO3-- from water
HC03- in the water reacts with the hydrogen and limestone (CaCO3) to form Ca2++ and HCO3-
Calcium is the plant food. And the HC03- buffers it back closer to 8.

IME, limestones (not powder) don't dissolve much in water with neutral pH around 7

 

[Alice B]

oh wow you are good at that! I just go with what I see, and I have pictures of limestone algae pits. It took almost 2 years for me to make the connection with incredibly tough algae being associated with limestone, particularly soft limestone, in the pond