Warning : the use of these tests must remain reasonable (once a week at most). It is necessary to avoid making any decision based on these tests alone because generally these tests lead beginners to carry out brutal and destabilizing actions for the baccalaureate. You have to learn to observe living things.

Details :
- below a KH of 3, the pH of the aquarium can fluctuate easily and weaken the fish. In the case of low KH, provision should be made to stabilize the pH by an appropriate means.

- plants need carbon to thrive. They usually use CO2 from fish respiration or from CO2 injection. In the event that the quantities of CO2 become insufficient, the plants will look for the carbon atom present in the hydrogen carbonate ion HCO3- responsible for KH. This results in a drop in KH which can become dangerous. This is particularly the case with powerful lighting combined with very insufficient changes or the absence of CO2 injection.

RO water: better to be interested in GH
Let us take the case of water at 10 ° KH and 10 ° GH. By using 60% reverse osmosis water during the changes, we obtain a KH of 4 but we also lower the GH to 4.
Adding reverse osmosis water lowers KH AND GH. However, if certain species like fresh water (low PH) they have difficulty withstanding too low a hardness (GH). They need the various mineral salts provided by the mains water, including calcium (one of the components of GH).
It is therefore necessary to take an interest in GH when considering the use of reverse osmosis water.

Conclusion
Be careful when buying your axolotls (or fish), you must be able to provide them with the appropriate parameters. The modification of the water parameters is often imposed on the aquarist.
These manipulations do not pose any problem with the means available. Especially since most species have fairly wide tolerance ranges, so it is often unnecessary to adjust the parameters to the nearest degree GH or KH.