Koi tank advice

[exee]

Hi all!

I've told my friend I'd help get his Koi tank back into shape for him, but I need some advice on how to go about cleaning this tank. I took my test kits over to his place to check the water and the results came out worse than I expected :P
The tank water has a green tinge to it.

The tank gallon size is about 60-70 gallons and houses 2 large Koi which have been in the tank since babies.

Ammonia : 8ppm + (off the scale)
Nitrite: 0
Nitate: 100 +(?) off the scale
Phosphate : 2ppm (not sure if this is really needed...)

The tank has a Aqua clear 500 running for a filter and a large pump/filter thing (not sure what its doing...) running from under the tank. There is no gravel in the tank. My experience is mainly with saltwater which is why I'm not sure how to proceed with cleaning up this tank. Does anyone have any suggestions on what I should do with regards to water changes etc.?

Thanks,

Eric