Hi, Davy,
sorry for the belated answer, but I have been away for a short holiday – and (what always keeps me from logging in here as often as in winter) – there’s so much work to do in the garden …………….
I use gardening peat (as described by Deepin Peat) to prepare the water for the tanks and the weekly water changes. I put about two handful of it into a nylon stocking and hang it in a 10 l watering can filled with rain water. I leave it there for about a week, until the next water change. It works only a few weeks, you have to control the pH from time to time. When the peat is washed out, you have to change it.
I use peat granules only as a thin ground layer in the tanks.