I have shredded peatmoss from the local garden center under both gravel and play sand in my two tanks. While I can't really swear by it or at it, I have found that it is a cheap "filler" and the plants do root well in it. That being said, you are looking at a long time for it to settle, your filter, if it's running at the time, clogging, and having cloudy water every water change until you vaccum the debris off the top of your other substrate. I have even added it while the fish are in there and, other than not seeing them for a while, haven't had any losses yet, touch wood. You will also have the blackwater effect from the tannins and it may drop the pH and soften the water like adding peat to a filter.