I’m just starting an unheated and unfiltered 10L Nano. The only occupants until this evening was some Tetra Active Substrate and four Moss balls. I bought some Spider wood to put into the tank today and when I took the cover glass off I noticed quite a bit of surface film. I’m sure the culprit must be the substrate as the Moss ball were happily sitting in a vase for a week without any problems. Anyone had this problem before? I did thoroughly rinse the substrate with hot water before putting it in.