My 180 gallon is so overstocked that I can't keep it clean - but I do perform weekly water changes on all of my tanks and don't notice any issues in tanks with normal stocking, such as my 120 gallon.
Fish can eat the beatles, they're the same as the mealworms really, just crunchy.
You really won't get too many, not all mealworms make it through the larval stage and pupate into beatles - and even then the beatles turn into cannibals, so you'll constantly have dead ones on your hands in the morning.
I just leave them in, and they eventually turn to mulch for the mealworms to eat again