Most veggies are fine, I blanch some of them to help them sink, but that is it. Don't cook them in the microwave, nor boil them for more than 30 seconds, they lose their vitamins that way.
i dont cook mine at all. potatoes just have uber amounts of starch so you just gotta dice em and rinse em really well, cause its bad for water quality (duno bout O2 levels). the scarlet will prefer more and more meaty foods as it gets bigger, but its always gotta be varied. prawns a good idea.
depends what you mean by huge. they get slow growers after the first half/three quarters of growth. years and years till full size. how big is it now? the better the water quality and a good varied diet, is the best way for quick growth