It depends on your style of watering. If you don't water often enough, I'd use straight long fiber spaghnum moss or rock wool (not the water repellant type). If you are a "regular waterer" or if you are worried about it being waterlogged, try adding some perlite, orchid bark and or pumice. Remember to use plastic pots only, since I found unglazed ceramic tend wick moisture away too fast and trap minerals and excess salts from the water. About 1 - 2 sizes up at the most. Good luck!