50 gallon drum, how many "aquarium gallons"

[mike31154]

You likely don't fill your fish tank right to the rim either. The larger your tank the more volume you lose with water level an inch or 2 below the rim. No voodoo here, sand, rock, ovreflows all displace water volume.

[mike31154]

I drain my 77 down about 6 1/2 inches from normal water level and that tends to be about a 22 to 24 gallon water change. No sump, no overflow. A skimmer feed box displaces a bit of water at the surface. Lots of rock & sand. 24 inch tall tank water level an inch & change from top.

[mike31154]

Also a remote possibility the difference between Imperial & US gallons may be a factor, depending on where the hardware is made.