Turtle questions :) ?

Well I have musk turtle perfectly round shell female I think? How do you tell? Shes i bit bigger than 3 inches And she lives in a 20 gallon tank tall filter about 10 inches of water and a foam dock in her tank ... Can put sand or small pebbles in her tank? Plants in the tank in the pebbles/sand? Add another turtle of the same or different type? And I feed her every 4 days good? Bad? Does she need a place to hide what else do I need since I only have 2 things in there .... Also I have a heat lamp but after the bulb went out I stopped using it cause it caused alge to grow but now that I've stop no alge :) .... My room is the hottest room in the house during the day so is this okay? Thank you!