Backstory: live in WI where water is fairly hard. Ph sits at 8.0, gh is high and kh is also around 150ppm out the tap after sodium water softener.
I set up a 210 gallon aquarium with a 40 gallon sump. Had crushed coral sand(mistake) and a mix of red granite gravel and volcanic planted substrate. And installed 2x 48" fluval 3.0s. With injected c02 via inline diffuser. Specific plants thrived! But most stem plants even when floated for 3 weeks would just melt after being put in my aquarium. I did numerous amounts of testing and found that in my tank my Gh was at well over 300 ppm.
Present:
I have now installed a rodi water system and removed all but a half pound of the crushed coral sand and added 60lbs of eco complete black aqua soil to the tank. I've done close to 40 to 50 gallons of water changes now removing old water and putting in pure rodi water. After testing it looks like the ph has dropped to 7.6, kh is at 120ppm but GH won't change and is still holding at over 300ppm. I don't know if I am fighting a losing battle or if I need to add tannins to the water to help in dissolving some of that gh.
I want more flexibility in what I plant in my tank. And hard water I feel is my culprit what are some thoughts on this?