water changes will help. i do a gallon water change out of my 5 gal fry tank and 2-3 gal out of my 20gal fry tank every 2-3 days. and i think a tank of platies would be awsome! i was going to do that when i first got my 20gal, but it was too expensive so i went to bid goldfish and now it is a fry tank! i personally would gat all kinds of platies so there is a lot of color variety or get a few kinds that would lokk good if bred, like white mickey mouse+ blue spot, sun set wag+ mickey mouse, etc about the brine shrimp- i dont know, i have never used them....