line current control ideas

Hello everybody its been a while since I've messed with my Arduino, lots of pressing things in life lately, but I'm getting back into it with an interesting project and I need to know if a certain device exists.

I need a device that can splice into a small motor's (actually an aquarium powerhead) power cord that allows an Arduino to PWM how much line current is going into the motor (powerhead).

The project is an arduino-based wave generator in the reef aquarium I'm building.

Any such beast exist? Also in case I decide to try running more things in my aquarium from an arduino, does the relay shield handle line current?

Thanks everybody!

We need to know about the motor. Is it ac, dc high voltage, low voltage?

Weedpharma

The motor hasn't been bought yet but I do know its 120VAC, line current, estimate of around 3.5 watts

Do you mean 3.5 Amps?

Weedpharma