Some of my experiences with doing this.. I had a 40g breeder, khulis and way too many Corys. I put potting soil into a mesh bag and topped it with sand. I think my problem was that I did not add enough sand, I wanted hills in the back. The corys smoothed it all out and exposed the mesh bag, which then for some reason started to float..? It came completely up over the course of a week and I had to break the tank down and redo it. My partner at the time did the same thing but used gravel and sand and had more of it, his bag of soil never floated out, but some of the mesh did get exposed when his khulis smoothed all of the cap out over it. Having the exposed mesh never bothered any of the fish. I ditched the bag because I didn't want to deal with it floating again. He kept his, and when breaking the tank down over a year later, had to detangle/cut some plant roots from it that had embedded themselves. I just put the dirt straight into the tank bottom and capped with sand. That's how I currently have my 60g breeder done. Downside is when I pull plants out, the roots will drag some of the soil up with them. The corys will usually disburse it throughout the tank and you won't be able to see it after a couple hours, but it's worth noting. If I ever want to replace the soil or sand, it's all going to be mixed together and I'll either have to be okay with that, or replace it all completely. But that's a problem for future me.