Way to go!!! Sorry for coming back in late to the larvae party.
A few things to check:
1. Aside from Gresham's advice about keeping the larvae away from the heater with a column of air, it is a good idea to use electrical tape to cover the heater indicator light, which tend to flock to.
2. Have you blacked out the sides of the hatch tank so that light comes in only from the top?
3. If there is too much air, the larvae get tossed around in the turbulence and don't hunt for food as effectively. I like to place airstones in the corner and leave calm spots in the center of the tank.
4. You don't have to use the greenwater method, but with so few larvae remaining the rotifers may take a while to be hunted down, in which case it may be beneficial to ensure that the rotifers are continuously being gut-loaded.
Best of luck with the wee ones!