Had a similar problem with marine clownfish, white poo, eating as you discribed. Treated with praziquantil. Not sure if it was liver flukes , internal parasites, internal worms etc but worked on one batch of about 100 fish.
You can get the liquid stuff but i prefer the tablet made into a gelatin based food mix.
The other thing to try is levamasol, this can be purchased from a large animal vet (sheep drench from memory.)