8x8x3 deep, feed em filter em n thats it but suppose next year out comes the shovel to make it bigger !! Wasnt that large till the three koi arrived, before that 4 c8" koi n 2 6" g/fish and my previous calculation wrong on size, just short of 1200 galls (750 was b4). Keep ya fountain running to aid aeration of the water, especially on warm days, believe me you wont stop at 4 small goldfish !!
A 'mature' pond will cope with 2/3" of fish (excluding tail) per sq foot of surface area, halve this figure to allow for growth.
But dont go too mad with water condition, i.e. don't keep changing it, if pond only a few weeks old or recently refilled it will take a while for all the nice chemicals the water boards put in to dissipate and by keep changing water you make matters worse!!
Just thought, we're now miles off the original question!! So hows the power box working Graeme ?