Sometimes when I feel enthusiastic I will have some nice organic greens, veggies, etc. I make little ice cubes of them - not too much spinach because of oxalic acid, just to be on the safe side. You can also add some calcium or Spirulina, other healthful additives. Easy enough to drop one in whenever you would want. You can get the ice cube tray which would suit your size tank - for big to the 1" tiny type!
Great advice from Junglefowl about not leaving them in too long.