I just picked up a brand new aquatools high rate fluidized sand filter model-at 3120. Although they are designed for swimming pools they make excellent aquarium filters- like a fluidized bed filter on Roids. This is a 20" unit and can hold 220 pounds of sand
This thing could easily filter a 2,000-5,000 gallon heavily stocked tank or pond. I bought it because I was thinking of converting it to Kaldness. I have seen it done and it looks even more efficient.
Question- use kaldness or silica sand? I haven't decided yet if I will even end up using it. I think it's overkill for my 1700 gallon tank. I might end up selling it to get a little smaller unit and just trying to decide if I want to open the box and fire it up and if so sand or Kaldness.

Question- use kaldness or silica sand? I haven't decided yet if I will even end up using it. I think it's overkill for my 1700 gallon tank. I might end up selling it to get a little smaller unit and just trying to decide if I want to open the box and fire it up and if so sand or Kaldness.