I used to work at an industrial-sized hydro greenhouse that also raised tilapia, and we routed the 'used' fish water to the plants.
Worked fine.
Granted it got VERY WELL OXYGENATED on the way.
You need to make sure there's plenty of O2 in the water so you get more nitrate and less nitrite, or worse, ammonia.

If you still have any doubts that you would like to use fish water, I strongly recommend using it to water your compost heap.

Storm Crow, how big has your plecostomus gotten with no competition? The biggest one I ever saw was about a foot long at Lyndon State College.