Well if my memory serves me right, you want to aim for at least around 20 - 30ppm or maybe higher for the plant's sake. I would just add enough GH booster to get yourself up around that level and see what happens. For a reference point, 1 tbsp of epsom salts will bring your magnesium level up to about 8.5ppm in 50 gallons of water. I'm not sure what the ratio is exactly for GH booster because I've never used it but I would imagine that you will need to add a couple of tablespoons to a tank that size if you are starting off with 0 GH. I would stop adding the epsom salts because I think GH booster already has the correct ratios to use by itself, adding more magnesium could throw it off and isn't needed. Then when you do a water change only add the appropriate amount of GH booster, so if you do a 50% water change add 1 tbsp of GH booster etc. to keep things stable for the fish.