"Courageous" did the calculations and came up with:

Okay, I'll assume an average depth of 2.25 feet. Your pond is
13 x 13 x 2.25 x 7.48 = 3,160 gallons.

Having done my own figuring and fellow ponder's figuring of their own ponds
and then running a water meter, or salt calculator, we've found all to be
off by 20-50%. All those folds, curves and I wouldn't be surprised plant
containers (if it is an older pond) take up space.

http://webpages.charter.net/gareee/Pond.jpg

So with the math and the picture above I'm betting she has something over
2,000 gallons, and the only way that little filter is gonna work is to mass
plant the pond. My lily pond of 1,000 gallons has 12 water lilies, 2 lotus,
4 baskets of iris, 2 baskets of arrowhead, 1 lizard tail and a few water
hyacinths, plus bunches of anacharis.

It looks like you have the starts of a water fall to the right? Do you have
anyone mechanically inclined to help you do an out-of-the-pond filter
perhaps? I'm thinking plumbing similar to what my guys did for me (see
diagram on webpage under *My Filter*) where you go over the side and use
siphon effect. www.jjspond.us ~ jan

~Power to the Porg, Flow On!~