Mosquito larvae are great for your fish. Depending on how far along
spring is where you live (here temperatures are just starting to creep
above freezing), you may already have hordes of mosquito larvae waiting
to be harvested in ponds and ditches around where you live.

Take care not to feed more than your fish will eat, and to store any
unused larvae cool, or they'll hatch into mosquitos in your house, and
you really don't want that.