I have always sand or some other rather fine ground material in my tanks, but not much – about 1/2 or 1 cm deep. I had never plants for decoration, but when they grow they can’t be so bad for the tanks biology …
The ground material also works as a filter – and does a bit of a self cleaning work, so that the ground don’t has to be cleaned all the time …
I test it before I use it – I fill a little amount in a glass with osmosis water and measure after some days if there is any Calcium. If not – good for the tank.
I have now four Paro tanks (36, 54 and 65 l) and the inhabitant rate is increasing 😛 so they like some plant structures I guess , even if it are just the roots of swimming Ceratopteris. My Valniseria also has adapted very well in the low pH tanks.