I have a Koi pond of approx. 22,000 gals. When I built the pond, I was advised by so-called experts, to install a bog type filter system. It is now 5 years old and the filter is plugged, to the degree that, there seems to be no filtration what-so-ever. Need some rock solid info on what type of filter system would be best for my situation without breaking the bank. I am retired, so I don't have unlimited funds to be throwing away. Would it make sense to dig up the bog (which contains 3 ft. of pea gravel) and replace the gravel with different kinds of filter media? If anyone out there can, please HELP!