i have frill in my guppy tank, yes i think it is normal for them to nibble at it as my smaller ones love it the older ones not so much tho. as fo a feeder plant i dont think so but goldfish do have loads of fun eating it up and leaving a bare stem. i dont think guppies actualy could eat its leafs . even if they do manage to eat leafs, dont worry about it it wont kill them.
