1. It has to be big enough for your turtle to move comfortably and to be able to hold enough water to cover your turtle's shell. I found that if The water was deep enough so that they could see the food from under water, that worked best.
2. If you feed pellets every day, then use it every day. If you feed them pellets every other day, then use it every other day
3. As the greens are usually left floating around so they can graze on them, it would be rather difficult to feed them out of the tank.. maybe others do feed them the greens there. Which is why I've not bothered feeding them separately.. as heck the greens make more of a mess than feeding them pellets.
I can get them to eat pellets out of the tank, but not greens, no matter how long they stay in the container.. so I gave up
