I haven’t figured out the pH / hardness issue exactly, but it doesn’t seem to be a real problem. I just get my calcium carbonate and potassium carbonate from a homebrew store for really cheap. I alternate adding one or the other with every other water top off. After experimenting, it appears to only take a couple tablespoons every few weeks to keep the water hardness above 40ppm. The pH stays at a constant 7.6, which the fish like, but I am sure this would not be the best pH for some plants. The mints, lettuces and basil seem to be perfectly happy with it though.
Here are a few pictures of it in June. I just keep giving it a trim and then it looks like this again 2 weeks later. We can hardly keep up eating enough salad and making enough pesto!!