The API test kit for GH, which I use and like, has directions that say 1 drop = 1°GH. But, what if you have zero GH?(see note 1) It isn’t going to turn color until you put a drop in. This was actually a problem for me because I let myself get persuaded to use one of the active soils. When they’re new, fight to keep the GH above zero can be tough. I was seeing my GH go from what I thought was 2° to 1°overnight, then realized the drop could actually be more than 1°! Measuring GH that is below 1 can be done, as well as measuring between degrees of hardness. I only ever measure to 1/3° now (1/3° is approximately 1 meq), and don’t do that often although I always test to the 1/2° after water changes. I can tell you how to do that with your API test kit if you want.
That said, before you put fish in there, you’re going to want to raise the GH. Like many, I have kept tetras and corys, which are “soft-water fish,” successfully in tanks that have water that is considered moderate to hard. Currently, I have Corydoras hasbrosus and dwarf neons happily inhabiting a tank with 4-5°GH. I keep it that high because I have mystery snails and snails keel over in water that is too soft. I also keep the pH up around 7.0 for the same reason. That tank also has one lonely Amano shrimp in it. The other three died before I found what works to keep the GH up.
To raise GH, there are a lot of products that can be used, with Seachem’s Replenish and Equilibrium probably the top two chemical additives. You can also use crushed coral or aragonite, but they’re hard to figure out and will raise the KH as well. Myself, I’m kind of a Seachem fan, as are many on the forums. I have found that Seachem Equilibrium works better to raise GH and last than their daughter company‘s Aqua Vitro Mineralize. I am not a chemist, so I can’t explain why the sulfates used in the Equilibrium last longer, meaning aren’t exhausted as fast, than the chlorides used in Mineralize. I think I’ve got it figured out in my head, but like I said, not a chemist and am old enough that in high school chemistry we still used the Bohr model.(see note 2) As for KH, although there are some posters that say to go ahead and use baking soda, I really don’t recommend it. Baking soda is sodium bicarbonate, which after reacting with an acid, liike the carbonic acid from your CO2, wil have one end product of salt (NaCl), which can build up if you have a few weeks in which you’re not diligent enough with your water changes and that’s not good for many fish—like corydoras for example.
I don’t know what plants you want to kee, but many can tolerate water that is moderate to slightly hard. I see a lot of posts on here from people who say they keep their GH at 1° and their pH at 6.4-6.6, but that’s not for everyone. I love my snails—I even name them!—and want to enjoy planted aquariums, but the fish and invertebrates are still the reason I’m in the hobby.
If you want the procedure to test <1° GH or KH, let me know. It’s easy; after all I figured it out.
1 The same goes for the KH test of course.
2 Less you think me too old a fossil, I am updated on the current understanding of electron energy levels.
