I won't contradict what has been said above (because it all sounds reasonable and I have no experience with ponds and pool filters that use sand), but I have used a swimming pool filter for a small koi pond, but the filter I used had what appeared to be plastic mesh panels that filtered the water. I don't recall the brand, but instead of the large plastic or fiberglass one-piece body, this filter had a rim around the "shoulder" of the filter's body where the top and bottom halves were bolted together. This gave access to the larger internal parts that were not accessible through the valve body neck. Other than that, it operated just like most pool filters, and was easily back-flushed to clean it. It filtered very well, and what came out of it during back-flushing... let's just say that the flowers loved it! I ran it with a 450 gal/hr submersible pump before the filter with no problem.
Like I said, though, this was not a sand type filter, so you can't go by how that worked, but if you could possibly find a swimming pool filter like that one, it would work. It was originally on an older, large above-ground swimming pool, so it might be hard to find one like it. But if you did, it would work very well, I can say from experience.
"May the Fish be with you."