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GH: Keep the fish happy. Research the fish species you are keeping, but as a VERY general guide most soft water fish thrive when the GH is under 5 German degrees of hardness, and under 3 for very soft water fish, especially wild caught.
Hard water fish GENERALLY thrive when the GH is over 9 German degrees of hardness, and values approaching 20 degrees seem just fine with some species.
Fish that have been captive bred for several generations are more accepting of conditions that are not within the narrow range where their ancestors came from.
Research your fish.

Within the appropriate range of GH for your fish there should be some of each Ca and Mg, but I do not know if the exact ratio is important.
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Most average aquarium plants are just fine in hard or soft water, and within that full range of GH that fish like, plants are usually just fine with. Plants seem to use Ca and Mg in a ratio somewhere around 3-4 parts Ca to 1 part Mg.
You can test the Ca, then use some math to figure out the Mg based on the GH and Ca readings. It is not simply GH - Ca = Mg, though.

There are some species of plants that are more particular. Research the plants you want to keep to see if you are dealing with one of the odd ones that really does need a specific value, or a narrower range of any of these minerals.

Most aquarium keeping is not so precise. If you are in the ballpark, most fish and plants will do fine with a wide range of levels.

Some plants use carbonates (KH) as a source of carbon. If I remember, though, they only do this if there is no CO2 available. Approximately half the aquarium plants are capable of this.

If your water has NO magnesium, I would dose some. Epsom salt is one possible source.
 
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