Lovely part of the world you live in. Afraid I have no idea of the water perameters. We use our own spring water. Changed most of the water at the beginning of April I think and the bulb and green mats have just been renewed. We have a pump which runs at given times and air bubbling tubes. Does this help?
Weve 27 years of koi keeping under the belt Pip pumps should be run 24/7 365 days in the year and I take it you mean airstones yes ?
The UV-C should start to kill off your algae but your filter by the sounds of it need to mature .
Whilst this is happening your waiting for Ammonia and Nitrites to spike which can be a bad old time for your koi so plenty of small water changes , the Peramters I asked you are important to the lives of your koi get things slightly out of kilter and your fish may fall ill or worse still may die , this is why we do the readings so that we can manipulatte their living enviroment should it be going wrong anywhere .
Next time your in town go into your local aquatic outlet and ask if the have either API pond test kits or Tetra Pond test kits in drop form not strip.
We are all here to help you Pip so if in doubt ask questions
Dave