I am using RO water for my newly started 100 liters aquarium and it results in a pH value of 6.0 to 6.2 which is too low considering that KH is 0.

In this case, I need something to raise pH (and KH eventually). The candidates for the time being are:

NaHCO3 (baking soda): Raises KH and pH but adds Na+ (sodium ions) to water which is not very useful.

KHCO3 (potassium bicarbonate): Raises KH and a tiny bit pH but needs to be used with care since adds K+ (potassium ions) in the water which is very useful in right amounts, but excessive use can be bad for fish and plants.

Do you have any other methods/formulas/combounds to raise pH in RO water in a controllable way without adding any unknown elements in my aquarium?

Thank you.

  • Seachem equilibrium uses: potassium sulfate, calcium sulfate, magnesium sulfate, ferric sulfate, manganese sulfate.The percentages are listed on the product. It works well, the raw ingredients are cheap and easy to obtain, I'd go with those.
    – Jestep
    May 16, 2019 at 14:15
  • Hi Jestep, I am aware of Seachem equilibrium but it is used mainly to raise to GH not the PH of the water..
    – Jibrilat
    May 16, 2019 at 14:24

4 Answers 4


You can use crushed sea shells or even better egg shells (egg shells needs to be dried in your oven at low temperature 60-70 °C before you crush them).

Another natural thing is to use marble (crushed or chipped to make it dissolve a little quicker).

It will take some time to get the pH up to 7,0-7,5 about a week or so. And when this pH is reached, the calcium carbonate will dissolve very slowly.

These things are the old way to get the pH up in a controlled way.

Sorry for the short answer.

  • Hey, How eggshells behave with the water? Do you have any source in order to see exactly the rate of PH and the elements that release? I read that adds Mg/Ca in the water but I don't have any good official source to cite. Thanks!
    – Jibrilat
    May 16, 2019 at 12:17
  • Well, seems that eggshells are made of 95% of CACO3 so it makes sense but the problem with CACO3 is that it is not soluble in the water, therefore, you cant create a fixed dose and aim for a specific PH increase.
    – Jibrilat
    May 16, 2019 at 12:57
  • 1
    @Jibrilat, Calcium Carbonate isn’t dissolving in water, though, it’s neutralising the dilute acid. You essentially "overdose", and when the pH is high enough, the calcium carbonate stops dissolving (so you can’t overdose). I’ve used coral gravel as a buffer in a soft water area to stop the pH plummeting.
    – Pam
    May 17, 2019 at 20:26

Have you tried a piece of limestone as a decoration? As long as it doesn't have anything toxic in the stone it should be good.


For salt water tanks and African cichlid tanks I use either crushed coral (limestone) or dolomnite (aragonite) gravel. Most pet shops will have it.

For salt tanks I have added agricultural lime (mixture of CaO and CaCO3) from any garden shop. You must have some alkalinity to buffer very clean water, not a problem with natural waters. I often throw a handful of aragonite into HOB filters to be sure of buffering. Usually a natural gravel will give the water some buffering.

I have made the mistake of putting rainwater in a gravel free tank and put fish in it temporarily - overnight, it killed a bunch of swordtails. The fish respiration added enough CO2 to acidify the pure/rain water with no buffering. Yes, fish put out CO2. A tilapia farm had 5 large (100 ft / 30.5 m diameter) ponds in a building to produce fish in the winter. People would get dizzy when they were in the building for a while. They found CO2 from the agitated ponds was replacing oxygen in the air. They fixed it with fans blowing air into building, no matter what the outside temperature.


To raise the pH value you can add baking soda, but only if the total alkalinity of the water is also low. If the alkalinity is in the proper range or if it's high, you should use soda ash to raise pH.

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