AC pump spd control? Cheap saltwater-safe DC pump?

I know that controlling the speed of an AC motor is "more challenging" than a DC motor, but is it really THAT tough?

The application I have in mind is for a variable speed pump on a saltwater fish tank. I'd like to be able to ramp a pump's speed up and down, but am having a lot of trouble finding a suitable DC pump, since everything in the aquarium hobby is designed to plug into a wall outlet and run at a constant speed. There are some "controllable" DC pumps for aquariums, but they are stupidly expensive.

So, how hard IS it to control a regular, AC motor in a pump? Or, should I continue searching for a DC pump that'll work?

Some examples. Here is a typical AC pump ("powerhead" in aquarium-hobby terms):

http://www.marinedepot.com/ps_ViewItem.aspx?category=Hydor_Koralia_Circulation_Pump_Powerhead_UL_Fixed_Flow_Aquarium_Powerheads&vendor=Hydor_USA&idProduct=HD19101&idCategory=FIPHFF

Here is a controllable, DC powerhead:

http://www.marinedepot.com/Tunze_Turbelle_Stream_2_Controllable_Pump_6105_(New_Generation_with_Titanium_Alloy_Shaft)_Adjustable_Flow_Aquarium_Powerheads-Tunze-TZ1343-FIPHAD-vi.html

Rather than spend the extra $150 - $200 for a powerhead with a DC motor in it, I'd love to be able to control the speed of the AC-motor-equipped powerhead with an arduino.

I've read a bit about AC motor speed control, and it seems like unless you have a 3-phase motor and a VFD, you're out of luck. Is this true?

You can control AC motors through thyristors (do some googling)

Some of the issues are that without the proper circuit you only get 0-50% speed.

But again, do some googling, you'll find what you need in terms of the overall process.

From the Arduino I believe you'll need to essentially create a 0-5V signal to geed the motor speed control circuit - at least that's one way...

good luck!

Have you considered something like this?

http://www.defender.com/product.jsp?path=-1|51|299222|84462|316441&id=1159977

You will find variations on these in any boat supply store.