PJ,
I have them (had until renovating this summer) in a less than 1/4 acre pond. They were just fine, in fact, really cleaned the little pond up like never before. The only thing to be concerned about, and it really isn't much of a concern, is if you do not have any predators in the pond, the Tilapia will present you with a pretty good fish kill, when the water turns cold....but Nature's clean-up crew, which works all the time in Texas, will quickly take care of the dead Tilapia.
