I had a regular pond filter on my pond and it was a lot of work to clean and maintain so I built a bog filter, it was only small but worked great and got the pond crystal clear in about a week. I think they’re the best filters for ponds and a 12 x 4 ft space would a great sized one for they pond. Pleanty of room. They say it has to be 30% of the ponds surface area in size for a heavily stocked pond so that should be a good size. They’re also cheap to build