RRR, there's a few tire shops around there that use a dense foam to fill the tires. It adds the same amount of weight to the tires as antifreeze or the water, but doesn't rust the rims and they still can dismount the tires if needed.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).