All snails will eat plant matter and as their poo breaks down it produces bacteria that encourages the growth of paramecium (infusoria). Unfortunately any filtration in the tank will quickly suck up the bacteria and infusoria leaving little if any in the water for the fry. If you only have a couple of fry in a well planted tank then there is usually sufficient infusoria and other micro-organisms growing in and around the plants. For large numbers of fry you need to culture infusoria in a separate container.
The best way to do this is to have a 40-60litre container full of freshwater. Add 1 whole green lettuce per 20litre of water, (rinse the lettuce well to remove any chemicals on it). Break the lettuce up before adding it to the container. Have an airstone bubbling away in the container and cover it up. After a few weeks the water should have cleared and if you remove the airstone and wait for about 10minutes you will see small clouds of white dots in the water. These are the paramecium and can be scooped out and added to a fry tank.
Once you get to this stage you need to add 1 or 2 lettuce leaves each day and add some fresh water to the container to replace the water that was remove with the paramecium. If you do this a culture can last for months.
Keep a cover of some sort on it otherwise mosquitoes will get in and their larvae will eat the infusoria.
