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Re: water displacement by substrate (was: NFC: Re: FilterlessAquariums.)

Someone called for lurkers ???

"Displacement" implies volume, not weight, so conversion of gallons to pounds (of kg) isn't exactly what we want. 

For a quick approximation, simply measure the (average) height of the substrate from the bottom of the tank (say 1 ") compared to height from the tank bottom to the water surface (say 10") and take the ratio ... in this case, 1  / 10 = 15%.  (Note: Assumes tank with vertical sides.) 

Aquatic plants probably account for an insignificant volume in a typical tank and can be ignored.

For more accurate estimation, fill a 1 gal pitcher or milk jug with the substrate material in the tank "up to the one gallon mark.". Then, using a measuring cup, see how much additional water you can add until the water is flush with the surface of your substrate. Say you can put in 12 oz of water. That would imply that each "gallon" of substrate actually displaces only (128-12)/128 or 90.63% of a gallon of water. Use this formula to refine your estimate per the earlier method.  (e.g., with my example numbers, 15% "visual displacement" adjusted by 90.6% "actual displacement per gallon of substrate" would yield an estimated net "non-water fill of the tank" of 13.6%).

If you need a more precise estimate than that, you're going to have to empty your tank, thoroughly dry your substrate, etc. etc. etc. (Or check in to your local Center for the Study & Treatment of Compulsive Disorders.) 


Doug Dame
Interlachen FL