OK, putting a valve in the line is like putting a band-aid on a broken leg, IMO. The pump's not going to be happy with all that back pressure, and obviously the tubing isn't either. In fact, you will significantly affect the life of the pump by restricting its flow like that.
The proper solution here is to either:
1. get a new pump that runs fewer GPH.
2. get a new overflow that can handle the existing pumps throughput.
I know people use valves like this, but that's not what they were really intended to do.