I bought a load of "rare earth" magnets for less than that.
Check out this one (might actually be too big):
http://www.gaussboys.com/product_inf...products_id=21
2 bucks, baby!
If you go larger than this make sure your kids don't get them. They will smash their hands to bits playing with these things. I am thinking this and some sylicone will do the job.
The two reasons I have not tried modding these for the tanks is:
1) time
2) not sure what it would do to magnetic drive pumps.
Now, thats worth a... pizza... cough cough. haha