I'd stay away from dimmer switches and AC motors.
You can control the speed of a March pump, to a degree, with a variable transformer. Trade names are Variac and such. I've got a couple of them, and use them on my pumps. The problem is that, if you slow the pump down too much (say less than 70% of full speed), they don't develop enough torque to pull the load. That's a good way to burn up a pump motor.
Another problem with the March pump in this application is that March pumps don't seem to have been made for continuous duty (by which I mean days). They get hot! And there's too much can go wrong to leave one running while you go to work.
I like peristaltic pumps for continuous duty. Here's an image from the wikipedia
Dirt simple, low maintenance, available in a variety of sizes and flow rates, and usually very expensive.