I looked into this once, to save money. The biggest problem is finding a powerful-enough ultrasound transducer. The next is mounting it to the tank effectively so that it will work.
I quickly found out that it was cheaper, and much easier, to buy a used on from eBay. There's not really much challenge, electronically, to making one so it's not a very interesting project, just difficult.
Do you really need ultrasound? What about a high frequency audio tweeter? Powerful ones can be had for little money and might do the job. Depends what you want to clean I suppose.
FoxyRick.