Mealworms are an acceptable adjunct to the diet of predatory fish, but they shouldn't be used too much. The main problem with mealworms is that the nutritionally incomplete. Unlike a good quality flake or pellet food, they don't have all the vitamins and calcium predatory fish need. When used to feed reptiles, they are always gut-loaded for a couple of days and then dusted with calcium powder (at least by responsible reptile keepers). There's a good explanation of the use of mealworms at the Drs Foster and Smith web site,
here.
Another issue with mealworms is that they are incredibly high in fat, with about 15% of their mass being fat.* This is way too high for most fish. It is now recognised that excessive fat levels cause problems for both marine and freshwater fish, and post-mortems reveal that among other things the fat accumulates around the internal organs. This is probably even more serious for omnivorous and herbivorous fish than it is for predatory fish, and I cannot stress strongly enough that fatty foods should be used very sparingly to feed non-predatory fish. For example, I've been told by one scientist examining (naturally wood-eating)
Panaque catfish that almost all the prematurely dead specimens he is given by aquarists have these fatty deposits around their organs. Bob Fenner has described something similar from large numbers of dead lionfish that had been fed goldfish, which are also fatty, so there's probably a close relationship between fat and mortality.
In short, while you might use them once in a while to feed predatory fish, they shouldn't be a staple food item. Offered once a week might be fine, but otherwise use a good quality pellet or flake food, or else safer live foods such as earthworms, snails or river shrimp. Wild oscars for example feed extensively on snails and crayfish, but never on goldfish, so why do people insist on giving them feeder fish?
Personally, I wouldn't use mealworms at all to feed anything omnivorous or herbivorous. With those fish, it's the green foods that make a difference between good health and poor health.
Cheers, Neale
*Waxworms contain even more.