First you need to calculate the volume of water, this is done by multiplying the length x breadth x depth if it is oblong or 22/7 x the radius squared x depth if round
i.e 2ft x 3ft x 3ft= 18 cu if oblong
or 22/7 x (3ft x 3ft )=28.25 if round with a radius of 3ft
1 cu ft of wateris equal to 6.24 gals
there fore the volumes of water are 18 x 6,24=112.32 gals or 28,25 x 6.24 gals= 176.28 gals
just alter the figures to suit you feature. once you know the total volume of water in your feature you can then check on the size of the pump depending on how quickly tou want to turn the water around