What are those little pots or whatever you have your plants in?
They're called net pots and come in different sizes. You can get them on Ebay. I think they originated with hydroponics. They hold media and the seedling to allow the root system to get going.
I was told to use a 70/30 ratio of coco coir and vermiculite in them, which was a mistake once they were moved to the raft. The movement of the water from the diffusers under the plants causes much of it to wash away. I had to repack and add coffee filters in the bottom of the net pots to keep it from washing away. I will use gravel next time or I will use cheese cloth around the coco coir/vermiculite.