I have two fish ponds. Both were set in concrete before I bought the house.
One is a long industrial time sink basin, perhaps 2'x4'x12", I keep only a couple of gold fish in there, and it has a submersible pump inside a filter box. I have to clean out the filter box maybe once a month. It took a while but it turned out a small magnetic impeller pump works best with the least energy consumption.
The big pond is perhaps 1,000 gallons, about 12' x 12' x 18", and has an external pump that is situated below water level outside the pond. This has a debris pre-filter in a chamber inside the pond that I can access from the pond edge. The pump then feeds into a modified swimming pool filter (filled with plastic roller curler type filter media) and then into a cascade above the pond that flows through dry ramen style filter media. Despite being packed with fish and a pond turtle the water stays clear, largely thanks to the ramen media filter on top of the waterfall.
The pump for the big pond is a Hayward brand spa pump. It has two speeds; I run it at the low speed 7x24, and then flick it to high speed only a few times a year when I clean out the modified swimming pool filter. I figure it is equivalent to the power consumption of a 150 watt light bulb when at the low speed. Unfortunately this particular two speed Hayward pump no longer seems to be available, but I'm told that by running it at low speed most of the time it should last nearly forever.
I have to clean out the pre-filter, however, about once a week during the summer and maybe every 2-3 weeks in the winter. It gets clogged with debris, mainly pieces of blanket weed that loves to grow in the pond (but which the fish largely keep under control).
My basic message is that even in a pond without fish and with minimal nutrient levels, you'll still have to clean out the pump filter on a regular basis. An external pump will be easier to design with a more readily accessible pre-filter than a submersible. But both will be a PITA if you're not into it.
My advice: if the tenant can't handle the pump/filter maintenance, don't do it. Put a bunch of nice looking plants in there instead, like water lilies, Japanese iris, etc.
Besides, I'm used to it now, but I once had a neighbor complain about the sound of the waterfall. A pond without a fountain will be very quiet.
Oh, and a 1.5 HP fountain pump will eat electricity like it's going out of style. I think the formula indicates about 750 watts per HP. So that would be like running a 1200 watt appliance 24 hours a day, seven days a week. At $.10/KWH, that would come out to over $1,000 year in electricity costs. Best get a two speed if you can find it, and run it a the low speed most of the time and at the high speed when you want to impress people. You don't want to run it intermittently with off times longer than an hour or so because then the bacteria that colonize the filter will die and stink up the place and probably turn the water green and slimy as well.
