if you had a blenny beforehand and it died, id lean more towards eating habits as the culprit. some blennies and dragonets are fairly hard to get to adjust to tank life. they dont always take to prep. foods and even when they do, they can possibly go back and forth. (im currently fighting hard to bulk-up my mandarin from near death starvation) keep a close eye on his belly and make sure his belly is bulging a lil bit. if you notice a sunken belly and it progressively gets worse, you need to find a beter source of food for him. (pods. algae)