My tapwater is practically RO (17-30 tds depending on the season) ... I add Seachem Equilibrium for GH and a light dose of API cichlid buffer to get KH up to 3. Using seachem's acid and alkaline buffer was too unstable for me, straight baking soda gets used up too quickly ... The cichlid buffer has been perfection. Totally stable and slightly alkaline with neutral substrate, totally stable and 6.8-7 with ADA aquasoil Amazonia.
There's also a lot of info online and in Diana Walstad's planted aquarium book, on a DIY mix of Epsom salt and baking soda for KH. Talk about cheap!
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