will it melt its a rubermaid 38qt its for a sick fish tank

You should be fine as long as the heater is not in direct contact with the side of the container.

it just has to have a tiny gap between the heater and the plastic.

As long as the heater doesn't make contact and the heaters wattage is appropriate for the size of tub there should be no problems.I myself have an emergency 2 gallon tub complete with 25 watt heater.I only use it when QT is in use and I need to isolate another fish.