There really is not much debate about hardness in freshwater tanks and therefore this article need not be long. In addition most of the texts you have read probably dealt with the carbonate hardness and not with the general hardness. The carbonate hardness also goes by the name of buffering capacity or alkalinity. All three terms refer to the same concentration.
Note that most fishes and most plants prefer soft to medium hard water (the exception are African Cichlids who prefer hard water of course).
The suggested levels for hardnesses are as follows:
- Freshwater fish-only tanks
- dKH or carbonate hardness: 7-8
- General or total hardness: 4 to 5, although it is not all that important in reality, except if you are into breeding. In this case the general hardness needs to be low and the temperature raised.
Tests exist for both dKH and GH or total hardness. Various companies have them available. Tetra makes excellent tests for freshwater fish and/or plant tanks. The key in most cases is to control and test your dKH on a regular basis. Do not worry overly about the GH. The dKH is what affects your pH the most, and also the amount of CO2 that will be available to your plants (if you have such a tank).
- Freshwater plant tanks
- dKH or carbonate hardness: 3 to max. 5.5/6
- General or total hardness: 2 to 3 maximum. Keep an eye on both as they need to be within these parameters.
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