They eat decaying vegetable matter. Try putting some peat moss or sphagmum moss in a yogurt container wet it till the water reaches the top and then pour out as much water as you can. Mix in some potato peel or cucumber peel and lay it on the soil preferably out of direct sunlight. Cover it with a rag or leaves. Come back in about 2 days (maybe longer you need the vegetable matter to start molding). You should find springtails, millipedes and pill bugs in the container. To get the springtails out fill the container up with room temp water and the springtails will float to the top. Use a spoon to scoop out the water with them floating in it. Put them in another container and start a new fresh culture. Wait till you get a substantail amount and then transport them to the tank the same way.