I have leaned away from supplements but in your case it may help. Many items in your diet contain high amounts of thiaminese which will bind vitamin B1 and proves deadly to eels within a couple of years and many times sooner. These include shrimp, minnows, sardines, and depending on the brand of silversides. As of my most recent research, San Francisco Bay brand is the only true silverside that does not contain thiaminese. You may want to inject some liquid B1 into those food items, you'll have to figure the dose on your own, but B1 is water soluble and will be safe. Your diet also lacks efa's, which can also prove deadly to eels within just a couple of years. If you can't get them to take some fatty fish, you could also inject some Brightwell Aminomega, or some may prefer selcon; human grade supplements can also be used. Aminomega does also contain thiamine(B1) as well as vit c, which is good for the immune system. That shark vit would be as good as any other choice of supplement, it also does contain thiamine as well as vit E. Injecting is a better vehicle vs soaking, has a better chance of actually getting there, and doesn't rinse into the tank causing issue. Inserting a pill into food chunks is also a vehicle. Supplementing can be tricky as the sources are not "natural" and whether they are absorbed at all is questionable, as well as toxicity. Any fat soluble vits should be used with care as these can present a toxic build up, water soluble vits are generally safe. Over time those that have relied on supplementation doesn't seem to fair any better than not using them at all. But just as the food being fed, designing a supplement regimen needs to be carefully prepared. Human grade seafood bought fresh and frozen in small batches always fair better, and supplementation should be carefully targeted to specific needs, as in B1 and efa's.
