20 ppm nitrAte is actually ideal for plant growth. So don't sweat that at all. A lot of people add nitrate into their tanks to keep the plants fertilized.
As for raising Kh and GH. I've seen suggestions on similar threads to add baking soda ( sodium bicarbonate) to raise Kh then add Epsom salts ( magnesium sulfate ) to achieve desired GH.
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I'll re paste this, I was just adding it to another thread ...
Diana Walstad's book suggests using Epsom salt and calcium chloride to raise GH.
The suggestion I'm interpreting is to not use only Epsom salt. Given the same holds for many animals, I'm not surprised there needs to be a balance between calcium and magnesium.
“start with calcium chloride (CaCl2), which is sold as a winter “de-icer”. It dissolves quickly, doesn’t contain sulfates, and won’t increase the pH. A level tablespoon dissolved in one cup water is a reasonable working Ca solution. This Ca solution diluted 1:500 in distilled water gave me a GH of 4. One tsp (teaspoon) added to one gal is about a 1:800 dilution, so I would add a tsp of the Ca solution to each gal of tank water and measure the resulting GH. "
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“For magnesium, you can use Epsom’s salts, which is MgSO4. A level tsp of Epsom’s salts in 2 cups water is a reasonable working solution. This Mg solution when diluted 1:500 in distilled water gave me a GH of 1 to 2. A reasonable scenario: the tank’s starting GH is about 5 (after the CaCl2 addition) and you boost the GH to 6 or 7 with the Mg solution. I would add just enough of the Mg solution to get a 1-2 GH increase.”
Excerpt From: Diana Louise Walstad. “Ecology of the Planted Aquarium.” Echinodorus. iBooks.
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https://itunes.apple.com/WebObjects/MZStore.woa/wa/viewBook?id=661029773
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