Bang couldn't you rig up like a trickle water exchange into a 10 gal or something in your lagoon so that none of the babies get out and not too many of the rotifers escape?
Here is another idea: put plastic wrap over the 10 gal., and cut holes in the plastic, then cover the holes with cheese cloth, so none of the babies or food escapes, but enough water can circulate and let light in. Then throw the whole tank completely submerged in the lagoon.