What your gh reading means
WatchBelow 4 dGH (under 71 ppm)
Very soft: good for soft-water fish; shrimp and snails need more minerals.
Good4 to 15 dGH (71 to 268 ppm)
Medium: suits most community fish, shrimp and snails.
WatchOver 15 dGH (over 268 ppm)
Hard: suits livebearers, rainbowfish and rift-lake cichlids; soft-water fish do poorly.
For a tank with fish. The same bands run in the test reading checker, which reads all your results together.
What GH is
Hard water picks up calcium and magnesium as it runs through limestone, chalk or gypsum. GH measures those minerals. Fish use them, shrimp and snails build their shells and skins from them, and plants need them as nutrients.
GH and KH are different
Test strips often list them side by side, but they measure different things. GH is calcium and magnesium; KH is carbonate, the buffer that holds the pH steady. Water can be high in one and low in the other, for example after a GH remineralizer that adds no carbonate. KH explained
Units
One degree of hardness (dGH) is 17.848 ppm (mg/L) as calcium carbonate. So 4 dGH is about 71 ppm, 8 dGH about 143 ppm and 15 dGH about 268 ppm. Test kits that count drops usually give one degree per drop; strips give ppm bands. The test reading checker takes either unit.
Which animals want what
Each care page gives its species’ hardness range. Cardinal tetras, discus and rams come from very soft rivers; mollies, platies and rift-lake cichlids from hard, alkaline water. Cherry shrimp molt badly in very soft water, and snail shells erode in it. Fish for soft water · Fish for hard water · Cherry shrimp
Changing it
To soften, mix tap water with RO or distilled water, which has almost no minerals, and test the mix until it reads what you want; always mix the same way so each water change matches. To harden, add a remineralizer made for aquariums to RO water, or crushed coral, which raises KH as well. Change hardness gradually, a few degrees at a time.