The biggiest headache I have discovered on ponds is having a proper and effective pre-filter setup so not to clog the pump setup. Filtration is key, I have succefully used one of the cheap drum style filters from Home Depot for years, it finally developed a leak, after ordering new gaskets/rings, still leaking, going to chunk it. I bougth one of the big mesh pre-filters from AC, and have yet to get a new filter, as it's end of summer...the pre-filter is doing a good job by itself...but I only have 5-7 fish (only one BIG butterfly koi)...it's a 60 gal, with waterfall... No water changes here...no issues. I would look into a surface skimming setup as well for pre-filtration...