How can I increase the carbonate hardness?

Lois, you want at least 50ppm (or 3 GDH, German degrees of hardness, it will depend on how your test kits reads it) of both KH & GH. My tap water is just about that so I rely on water changes mostly . But at my old house it was lower & I added crushed coral to the filter. I never had it completely dissolve over several years. I used ~2 TBSPs in a 20g tank. Check your GH too, usually, but not always low KH =low GH. & your water may change over seasons too, depending on where your water comes from, it changed where I used to live. Let your tap water sit out in a glass or bowl before testing pH. Many water co. raise it temporarily to avoid wear on supply lines.

I too, bought a giant bag of crushed coral & have enough for me & everyone I know, lol, TTA. I should test (everything) more often especially since I have a lot of plants these days. There are always things we can do better...I still need to read several articles you've posted.
 
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.

Since you have a 60L tank, you would add .30 teaspoon for each dH you want to raise it. You may end up with a pH in the 8.2 range. The problem with using this is a build up of sodium can be fatal to many fw fish. I would strongly suggest you go with the coral if possible. It is simple and has a wider margin of error than baking soda.

Calcium chloride is a PITA to dissolve unless there is acid involved. I used to start with 20 gal. (76L) of tap water at pH 7.0+ which was run through a few cups of crushed coral in a H.O.T. Magnum for 24 hours. The coral raised the GH/TDS between 15 and 20 ppm. I then added a couple+ teaspoons of Epsom and it raised the TDS by about another 75-80 ppm. Finally, added about 1/4 teaspoon of sodium bicarb to goose a few more ppm of TDS without raising the pH.
 
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