I've kept a lot of severums over a lot of years-- they
like veggies, but with a good staple pellet they hardly
need them.
Also:
1) You can overdo veggies to the point they don't get a good balance of nutrients, including sufficient protein. In fact, you can inadvertently create a bad combination of plant based products for fish--
link.
2) Don't treat them like goldfish or algae eaters. They don't need algae wafers or veggie pellets. Including some good quality algae or veggie foods is ok, but check the ingredients, there are a lot of products out there labeled as 'algae' or 'veggie' foods with an abundance of grains, starches, and other cheap ingredients, and they won't bring out the best in them.
NLS isn't the only food I like or use, but ime severums like it and look good on it, including NLS Thera-A.