In my above post I mentioned that a rubber maid container can be used as a tank. However if you want to use a fish tank I wouldnt get anything smaller than a 10 gal. Bare bottom tank no substrate material with some sort of fake coral or something your fish can hide in and feel safe.
No it doesnt have to be lit but its a good idea, as this tank is also used to get your fish used to your light cycle, you in general, your voice, feeding schedule etc. The more comfortable they are the better they will adapt to the DT when its time to put them in it.