I've used Jiffy Pellets. What I did was soak them in water until they were completely moist, then I squeezed some of the compost out of the top end, being careful not to tear the nylon that it is contained in. Squeeze out about 3/4 of the Jiffy Pellet compost and then shape the remainder of it into the plastic AG pod making sure to keep the bottom of the nylon on the bottom of the pod. Make sure the compost isn't too tight in there and then make a hole using a toothpick or something similar and put your seedling down into it. I have three like that. After I did that, I just put the cover back over the top and put it into the AG. Worked like a charm. Didn't even have a problem with the compost falling into the reservoir. And you don't get that mildew, moldy smell that some sponges get if they aren't the original AG sponges.