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Please ignore the first sentence of my original reply - it's cobblers!
It doesn't make any difference to the gallonage of the pond, but just to clarify what I should have said: The second dial from the left reads thousandths of a cubic meter, not thousandths of a litre - i.e. it reads litres, not millilitres.
So, to the nearest litre, the meter is reading 21,680 litres (21.680 cubic meters or 21.680 tonnes).
I've found the easiest way to read these is in litres and don't take the nearest number to the pointer or centre of the display, take the lower number that has definitely been passed.
'Frinstance:
On the display which gives m3 (1 m3 = 1,000 litres):
The first 2 has definitely been passed because the 3 is just creeping up from below. The first digit is 2.
The second 2 isn't quite in place, the 1 hasn't completely gone. The second digit is 1.
On the dials, reading from right to left:
The 6 has been passed, the 7 has been passed and the 9 has been passed. Ignore the last dial, even if it wasn't zero, it's tenths of a litre - (even I wouldn't measure a pond volume that accurately).
So you have 21,679 litres.
Divide by 4.55 and you get 4,674 (and a bit) gallons.