If you wish to have them just offer biological filtration and are not trying to keep the pump from clogging, yes. A pre-filter is typically to prevent the pump from clogging. Bio balls are NOT that easy to clean either, in my experience. I have used them a lot, and the sludge/algae gets trapped in them, and instead of having one sponge to clean, you have to hose out each individual ball. Much harder than a sponge. Most likely yours is difficult because the pond is overstocked. An average adult koi requires 500-1000 gallons per fish, and you indicated you have a "small koi pond". There should never be a small koi pond. Koi ponds should always be several thousand gallons because of the size the fish grow to so you may be in need of a larger filter, if you do not have one on the pond besides this one.
Date published: 2017-10-18