there should be a cold and hot level mark molded into the tank. you deff should not be seeing the bottom of the tank. if your engine has low coolant then that would explain the heat issue as there isnt enough coolant to flow thru the heater core unless you are stepping on it and the pump is pushing more coolant.

you need to fill the system up at the radiator and find where the leak is.

you can use the coolant level as an indication of how high the leak is off the ground... as the coolant level will be just under the level of the leak.