saltwater fish drink the water so they'll ingest some.. not sure it'd really be effective tho. in a dt i'd think it'd be really ineffective since established biofilters break down medications really quickly
metrodanizole is thought to mostly be reef safe, but ymmv. it hasn't been widely used in reefs like praziquantel. i think some people were doing fairly high doses trying to fight dinos on r2r with no issues, but again not sure i'd trust it. i know fenbendazole is not reef safe which is kind of the go to if metro + prazi doesn't fix internal issues, but it's harder to use
what have you tried to bind? i normally just do something like mysis or calanus and some tank water, focus + GC and the fish always eat it if they normally eat the food. some people do dry foods and it seems to suck up the meds better, but i've never really come across a bothersome internal issue myself where a couple weeks of gc + focus food didn't do the trick, but i'm sure they're out there