Just thought I would mention that most electrical equipment is not designed to work with a dimmer switch. Lights that work like that are specifically designed to do that.
For instance, let's say you put a normal bulb in a lamp that is designed to work with with a 3-way bulb.
Click 1: Not enough current, the bulb won't light up at all.
Click 2: Now there is enought current, the light comes on.
Click 3: More current, but the bulb was not designed to light up more than one setting, so the amount of light doesn't change.
I guess my point is that reducing the amout of current flowing to a pump that was not designed to operate at dynamic levels of current would probably not work, and might even damage the equipment. Just like the light bulb, the pump would not operate at low levels of current, and as you turned up the dimmer switch, as soon as it had enough current, it would start operating at full power. I wouldn't reccommend this option at all.
38 Gallon (Freshwater): 1 Green Swordtail, 3 Yo-Yo Loaches, 1 Clown Pleco, 1 Blue Gourami, 4 Otocinclus, 7 Cherry Barbs.
46 Gallon (Saltwater): 60 pounds Lalo Live Rock, 60 pounds aragonite sand, 1 Coral Beauty Angelfish, 1 Peppermint Shrimp, 1 Mushroom Coral, 1 Emerald Crab, various snails and hermit crabs.
10 Gallon (Saltwater quarantine):