A little background: my turtle is a rescue, his previous owners were friends of mine, and they were talking one day about how they thought it was funny to put their turtle in a hot tub, and how they had done it before
I bought him a 36X18X16 tank (40L I believe), filled it midway up, installed an AuqaTop CF300 canister filter (designed to filter 100+L I think? It's huge), and he has a plant to hide in and a large basking dock that floats, but is attached to the side of the tank. He doesn't have a heater because I was told that if the room temp is kept at 20C, they don't need one.
So my questions:
(1)
I was out buying a new bulb for my red eared slider today and the lady at Petsmart told me a plain basking bulb was fine, and that was all he needed. I have been buying Mercury vapor bulbs because they have UVA and UVB, is this the way to go?
(2)
I have gravel in his tank now but I'm not sure it should be in there? If not, what should I replace it with?
(3)
He likes to get out and walk around. When I first got him he walked with his feet curled over (similar to how an ape walks with their knuckles on the ground). It took a lot of gentle correcting but now he walks normally (and he's FAST when he's on carpet), and he really loves being out of his tank. Is it harmful to let him walk around for hours at a time?
(4)
He has little divots in his shell. I think they may be from the way his previous owners treated him. He has been shedding flakes (maybe 1-2 every few weeks?) and I brush his shell with a baby tooth brush (he seems to really enjoy it haha), but should I be concerned about the divots? Will they go away as he sheds?
(5)
He flat-out refuses to eat anything but pellets (his mix is a "gourmet" one containing what look like berries, and meal worms). I've tried feeder fish and they ended up just living in the tank with him (I went and got food for them too because he wasn't going to eat them and I wasn't about to let them starve to death). He will not eat lettuce/kale/greens of any kind, or berries/fresh fruit. I have tried and tried and he just will not eat any of it. What can I do? Are pellets enough? Does he need a supplement?
Thank you for your guidance. I will do whatever is best for this little guy. He has had a rough start to his life but he is safe with me now and I will do everything I can to make sure he is happy and healthy.

