We have always used a mild dish-washing liquid mixture to get rid of aphids, but I would think that wouldn't work well with your turtles We also have (in the dirt garden) planted marigolds and other plants that will drive away the aphids and such (it didn't do anything for the grasshoppers last year, they ate the marigolds too!)
I know this isn't much help, but I thought I'd put it out there for you to consider anyways.
I have the Urban Aquaponics Manual by Gary Donaldson - from down under. He just says to spray them off with an organic spray (if you have an open-loop system) - I guess that means plain water if you don't.
Originally Posted by Urban Aquaponics Manual, pp.87-88