A sand filter is not recommended for ponds. It simply cannot manage the quantity of waste put out by common fish loads. You don't mention how many gallons in the pond and that is the critical element we need to know to give you any advice but if you can only filter 1200 gallons in 24 hours that means the pond can be only 50 gallons. My pump for 1000 gallons puts out over 1500 gallons an hour with an additional pump for a skimmer/UV pumping another 1000 gallons an hour. Sandy

A slow sand filter would supplement the filtration of a 1200 gallon pond. The currrent 3 stage filter is 310 gallons at 1700 gph and skimmer at 1000 gph via venturi which is independedt of the filter. The Gph for slow sand is entirely diiferent than what you may be accustomed to. I would intent to divert pre filtered water into it at the much lower rate neccesary for the sand filter to function.

Would you be introducing the water from the top or the bottom. I suddenly had this image of quicksand.
I'm afraid you are likely to be on your own here. The best I can suggest is doing a search and I'm sure that is not what you want. In the past this subject has always had a negative response. At least, no one has come up with an effective sand filter in the time I have been on the forum. Of course there are others who have been here much longer but I haven't seen any of them posting anything positive either. I would be interested in what you find out if you succeed or not. After all, negative info still teaches something. Sandy

Spongbob, you would be as well off using polyester quilt batting as a final screening filter medium.
As Sandy mentioned, sand screening is not usually used in pond filtration and quilt batting is much easier, by far, to change out, than heavy wet sand.
Even lava rock would be easier to use, but I would not recommend it, as it plugs up and is difficult to clean properly. It also allows excessive channeling of the water around it, as it plugs up.
"Horton"

But yours is the final call on what you wish to do.

I have had a slow sand filter hooked up to a small test pond for 4 months. It has birds and raccoons using it regularly; and has all sorts of organic matter including leaves and small twigs at the bottom. The water has not been changed (except for rain). There are pictures at the link. It seems to be working very well. The water is very clean looking, a test for coliform has not gone in yet. My guess is that there will not be any contamination.

Here is a link that might be useful: pond filter

My question for you, Dave, is whether you have fish in the pond or not? It makes a big difference.

Excellent point. I would say it would be a tremendous difference depending on how many fish are in the water. One or two goldfish would probably not make any difference. I am sure that there will be a limit that a single slow sand filter can handle. The pond is about 5 feet wide and about 8 inches deep so it won't handle many fish. That said, there is a lot, and I mean a lot, of decaying organic material in the bottom of the pond now . . . but it only gets stirred up when the raccoons go swimming. . . and they do - mamas and babies - and the birds use it every day - for everything. . . but I know having fish in a small pond like this will put a major strain on a small slow sand filter. The questions I have are how much strain? and; Has anyone ever tried it before?

Here is a link that might be useful: pond filter

Sorry, I forgot to clarify: no fish are in the pond.

LOL, don't be too sure about that. Goldfish in a pond have a startling ability to grow. About 6 to 8 inches can be expected if the pond has enough food. Those ten cent feeder fish can get big.