Looks like early Hole In the Head/Lateral Line disease to me. Lateral Line Effusion is just another name for it.
It's commonly seen with overcrowding, over feeding and infrequent water changes, but your set up seems to be acceptable. There are also theories linking it to vitamin or mineral deficiencies, stress, and Hexamita infections. That's why you supplier made the suggestions he did.
If you Google Hole in the Head disease or do a search on this forum, you could spend hours researching it.
It may respond to the Metroplex and vitamins and minerals you're using. But it may not. It's a frustrating disease to treat.
The only other thing I might recommend, if you can manage it, is larger daily water changes. Up to 75-80%. Plus, I would only feed beef heart twice daily and siphon out any uneaten food within a few minutes. Uneaten beef heart pollutes the water column pretty fast. Try getting them on a high quality flake or pellet food to replace some to the beef heart feedings. Flake and pellet diets are a lot cleaner.