Underneath all that there may be some biomedia (plastic shapes of some sort), these hold bacteria that help get ammonia and nitrites out of the water and musn't be left to dry or cleaned in tap water or the bacteria will die.

You need to clean it when it clogs up or it might overflow, hopefully someone here has more info on the filter. Koi take some looking after but learning about the filter is a good start.

Basically keeping koi is keeping water, you need to keep the water quality good or they will get ill. So getting the filter running is the first thing. I wouldn't feed them until you've got it cleaned properly and would maybe buy a test kit and see if there's any ammonia or nitrite build up in there.

I feed my koi three times a day but have finally got a filtration system I'm happy with and it keeps everything clean. Is there a fish or koi shop nearby you can go for advice and to just look around and see if they have your filter?

The UV tube will probebly need changing too, they only last a year at best, so if you are lucky you might get two full summers per bulb, it dosnt need to be on in the winter, but the pump and filter will need to be left running, when you change UV tube be careful of the clips that hold it in, break them and you might be in a trouble.

Also as Mark says, for koi you keep the water, if the water is ok the koi are too, one important weapon in your kit is water test kit, for ammonia, nitrite, nitrate and PH they are the basic water test you need, feeding, well you will find a feeding rutine that works for you, but I find they fare better if fed little and often, I feed moine 3-4 times a day sometimes, depending if I am here to do it, if not they get enough to so they eat it all in 5 mintues, you will learn your kois feeding habbits in time, dont feed them when the water temp drops below 50f/10c.