Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Hard water is water with a lot of minerals.
We measure Ca and Mg as GH.
We measure carbonates and bicarbonates as KH.
All the other minerals and salts can be estimated with a TDS meter.
Soft water is water with low levels of all these.
To make hard water into soft water you need to remove the minerals and salts.
The best way to do this is with reverse osmosis or distilling. (There are systems that do both). As far as aquarium use goes, these are essentially the same. So where I write 'RO' I actually mean either or both treatments.
RO can be very efficient, removing close to 100% of everything (hospital level RO equipment, used for kidney dialysis and other procedures).
Home/hobby level RO is not quite that efficient, but you can get models that are not too far off 99% efficient, and 95% is adequate for what we do.
Fish and plants cannot live in water this pure.
You do need to add some minerals and salts for them.
The amount you add depends on the species, where they or their ancestors came from.
Most of the minerals in natural water came from limestone and related materials, dissolving as the rains wash over and through rocks and soils.
Limestone and its relatives are pretty much pure calcium carbonate, magnesium carbonate and similar minerals.
So, the streams, rivers and lakes start gaining Ca, Mg and carbonates. That is the things we measure as GH and KH. Traces of other things end up in there, too.
In high rain areas the minerals are highly diluted, that is, soft water. (Amazon, Congo and several Asian river systems)
In areas with granite or other rock that does not dissolve well the water will be fairly soft. (Many temperate zone river systems, including major parts of California)
In areas with a lot of limestone the water tends to be harder. (Texas Holey Rock & Kharst are examples of this type of rock. Well water is often high in minerals where the water table is in limestone)
In areas with a lot of evaporation, and low rainfall the minerals accumulate in the lakes as the water evaporates. Also, places that geologically came from coral reefs: Deserts, Rift Lakes of Africa, many streams and rivers in Australia, also many of the Caribbean Islands, Florida.
The extreme example of the last is the ocean. It collects all the minerals from all over the world, and LOTS of water evaporates! Similarly, salt lakes have a lot of minerals from the surrounding soils.
Research the fish.
Many of the soft water fish have been bred in captivity for so long they can live in water that is much harder than their ancestors. Wild caught, or just a couple of generations away from wild usually need the water to be more like where their grandparents came from.
Similarly, hard water fish might also be kept in somewhat softer water.
In choosing fish for a community tank it is safer to select fish with similar requirements. If you mix fish with very different needs, with just a narrow range of overlapping requirements you might be walking a tightrope to keep the parameters exactly in the middle, no leeway.
By selecting fish with similar requirements, and similar range of needs, the water can vary a bit, and still be well within the safe range for all the fish.