I just filled my 37gl again after a break from the hobby the last few years. I have a bucket that has gl measurement marks for 1 and 2 gl. I did it myself so its fairly accurate. So I filled the tank 2gl at a time.
To my surprise at 27gl it was at the bottom of the plastic frame and full. I have about 1,5 inches of gravel a good sized piece of driftwood and a 6 inch dia lava lock, plus a few hollow plastic aquarium decorations.
Surfing the net I find out that Aquarium company's measure tank volume using outside dimensions and to the vary top. So I measured from the inside and used the height of the water level it will normally be at not the top.
I came up with 31.5gl. Not even close to 37gl. I'm guessing the difference between 31.5 and 27 is the gravel and decorations displacing water.
The formula Using inches. L x W x H x 0.00433 = gallons
Maybe everyone should know the actual volume of their tank for adding chemicals, medications or figuring the fish load of the tank. I'm sure it will vary by size and maker.
happy fish watching.
A 37 GL but its really 27gl when full
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