Dave B,
Have you recently fertilized or otherwise had a slug of nutrients added to your water...maybe a big runoff?
Don't expect an overnight miracle with the Tilapia. It took a while to grow all that algae and it will take the Tilapia a while to clean it up, but they will indeed clean it up over time.
They are more effective at getting the algae before it floats, not after. For the most part, I've seen them eat the algae on the bottom before it is released to the top, but this weekend I observed Tilapia feeding on the floating stuff also.
