Fish can transmit parasites. Especially feeder fish.
Feeder fish:
Goldfish<---Terrible(they are high in fat and low on protein)
Minnows(not that bad a feeder)
Guppies(same as above)
Platies(same)
But all of these ussually contain parasites. Most of them die within the day. I used to work in a petstore and Id see 20feeders die a day. Even if kept in optimal conditions.