Hardness (and its opposite number, Softness) are really two sides of the same coin.

Both are due to dissolved minerals in the water any, or all, of which can have some effect on water quality and consequently on fishes, especially when they are transferred between two bodies of water which have widely differing characteristics without due re-acclimatisation. Development of fish eggs may also be affected if the water chemistry is wrong; for instance, Characins' eggs fair much better in soft acid water than is hard alkaline conditions.

A generalisation is that most 'hard' waters are alkaline (pH above 7) and 'soft' waters are acidic (pH below 7),
although you will find the odd exception to this - see More About pH.

The hardness of water depends on several factors:

Once in the aquarium, these factors come into play:


Like pH, testing for hardness involves mixing re-agents with a sample of aquarium water. Often, a colour comparison chart is used to read off the 'degrees of Hardness' or you may be required to count the drops used of the re-agent to effect a certain colour change in the test phial. Again, at the top end of the scale, there are electronic meters that can be used.


The simplest way is by dilution with a'softer' water. For instance, adding a litre of water of 5 degrees of hardness to a litre of water of 10 degrees of hardness will give two litres of water of 7.5 degrees of hardness. By mixing waters of suitable hardnesses in the correct proportions any hardness value can be achieved.

Ion exchange resins can be used as can reverse-osmosis (R.O.) units.

Note that distilled, or R.O. water alone (neutral in pH and no hardness) is unsuitable for fishkeeping.
Sometimes running water through a peat-filled filter will soften water, as well as acidifying it, however the degree of effect depends on the hardness/acidity of the original water.

Sometimes, fish from soft waters need to be 'changed over' to harder water for convenience of keeping. This can achieved quite naturally with little stress to the fish.

Simply set up an aquarium with ordinary gravel substrate and fill with soft water, as required by the fish.
Over the next few weeks, the water in the tank will gradually harden due to the effect of the (probably) calcareous substrate.
In the meantime, make sure that partial water changes are made using the hard water to which you wish to acclimatise the fish.
Eventually, the fish will find themselves living quite happily in much harder water as will any offspring from spawnings who will be used to hard water immediately - without any re-acclimatisation being necessary.

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