With the metal tank I would insulate it with polystyrene on the inside and add a pond liner as the metal of the tank is likely to cause all sorts of problems. Rays are affected by small amounts of metal in the tank in that it can kill them AND it messes with their receptors.
I think you will find that an RTC will also have a problem with Metal in direct contact with the water.
The insulation will help with heat loss and stop the liner being damaged.
Remember to keep the heaters in the filter as an RTC WILL burn itself on them and eventually try to eat it anyway.
One good way to filter is to add a (protected) pump in the tank and then pump up to a pond pump and then let gravity return it.
