But how much variety do my fish need?
Buying a dozen different brands and types of fish food does not necessarily provide variety. You need to read those labels. Chances are fish are still deprived of vital nutrients as a lot of fish food sold is a good as feeding McDonald;s burgers, and some fish foods may even contain harmful to fish ingredients.
If you want variety, buy top of the market fish foods, New Life Spectrum specifically. There are a couple of other brands like Omega and Northfin but in my opinion not as good.
Frozen stuff and the likes are only treats, and in fact not as nutritionally comprehensive as high quality pellet food.
I've exclusively fed my fish New Life Spectrum over the last 6 or so years. The NLS food is very comprehensive ingredients wise and is optimal for majority of fish we keep. My bristlenose pleco has rarely seen vegetables or has had access to any major algae. Feeding him veg once a month is an overstatement. He gobbles NLS pellets as the rest of my fish. He's around 6 years old, so are the majority of my clown loaches, I have 7-8 years old corydoras, 5 year old kuhli loaches, etc...I totally can't complain to sticking to one food only. I had a guppy live 4 years and majority of my platies surpassed that age. I wiped out the remainder via a broken tank a year and a half ago. Besides living long enough lives, they also display great colours.
I normally buy just two of their formulas, Cichlids formula(and I don't keep cichlids) and Algae max formula. I mix them in a third jar , tuck the remainder in the freezer, and I feed daily...I occasionally give frozen foods, once a week the most ....not for any nutritional benefits, just as a treat.