Actually in this case hardness doesnt matter. Thus why i wasnt worried about it for plants. From reading the OP's original post he isnt doing live plants in the tank due to the inhabitants living in the tank and he has tried them in the sump but the issue there is the flow so it makes it hard for them to work in there. Just an algae issue. He also mentions lots of issues with light. Thus why im curious about if he runs lights on the tank. If the light coming in the windows are there for enough hours and intense enough and then the OP runs lights on top of that during that time or when it gets dark he turns on the lights for a few more hours, your going to end up having a algae issue no matter what your Phosphates are at.
As far as hardness and plants, it can be done. Ive got several tanks I had that were on well water. GH around 18 and KH around 15 and I grew plants in it just fine. Of course there are some plants that did better and some that didnt. BUT still I had thriving tanks low light and high light with Co2 and doing dry ferts EI dosing method.