Well it's not going to be a HiFi speaker for long.
Speakers are pretty inefficient at converting electrical energy into sound energy, particularly the average home speaker. In addition the tweeter's power handling is very restricted. Add to that trying to get the sound energy into a tank of water and...No chance.
Ultrasonic cleaners on the other hand employ efficient transducers, directly coupled into the side of the tank and operating at high power, far higher than you would ever want to pump into a speaker.
As DTJ says, you can get kits, although an off the shelf cleaner may sadly be your cheapest option.
EDIT: Given the size you're wanting, it will need to be a powerful cleaner - you might do best looking for a second-hand industrial one.