From your description it sounds very similar to what I had happening in my tank. I had a 60G hex tank so it was pretty tall. If it is a matter of where you can see these particles just floating around in a circle then you need to change the water flow in the tank. If you have an overflow then you need to change it so it gets aimed at that more. If you dont have an overflow then it would seem your method of filtration is not picking up enough of them. Like if you had a HOB kind of filter. Then it is possible the water flow is not getting to that properly. That's my 2 C's worth. Might only be worth 1 C nowadays though.