Marineland (Aquaria) MLAND Repl Bio-Wheel Assy-99B 1 5/8" long x 2" wide Replacement of this part should only be done if the bio-wheel will no longer spin or if the wheel is broken or damaged. Discoloration of the pleated Bio-Wheel material is normal and desirable. And a slight irregularity in the Bio-Wheels rotation is normal. Rotating at any speed the wheel is working to eliminate toxic ammonia and nitrite.