From what I recall (I haven't personally set up a TEK light with brackets, I've only ever hung them), the brackets are not extendable, they fit firmly on the end of the fixture, so I guess they will only sit on the tank if it is the length of the fixture. That being said, you may be able to rig it somehow to fit.
There is a picture of them here:
http://www.jlaquatics.com/product/sl...acket+Kit.html