Depends on the fish your feeding, and whether that is in their natural diet and if they can digest it..
Also might introduce pathogens into the tank.. Although the Lizard might have built immunities to certain diseases, it could be a courier and introduce a north american parasite that has never met a south american fish ~ bad.
Some reptiles secrete toxins, especially if harvest fresh out of the wild, the toxins would have their greatest potency.
Over in asia they use frogs, geckos, channa and dojo loaches as feeders.. Seems to work good too, I always notice the arowanas being fed frogs and dojo loaches seem to be much healthier than the american aros that get comets and frozens..