They use them alot in the aquaculture industry. You cant usually use one of the cheap pre made poly tanks of varoius sizes because they are too thin walled. If you get a poly tank it has to be custom welded out of sheet poly with an acrylic panel inserted very securly. The pre made tanks bow like crazy! even the 1/2" wall tanks need an outside frame. Poly is great for big sumps where a little bowing isnt a problem. If you want to make your own larger tank out of something other than glass or all acrylic, learn how to work with fiberglass and insert a acrylic veiwing window.