I've been having problems with the koi pond the last couple of years, the pH has been too high, not being able to keep the algae growth down, etc.  I think I found the issue....I went to the local koi place to pick up more algaecide, and another customer asked how often she should replace her UV light bulb.  Replace the bulb??? It needs that??  Who knew (or even thought about it - the bulbs are hidden in the filter so you can't see if they are working or not).  I didn't even know where the UV fixture was in the pond, I just knew that Ralph had added one.  A quick chat with the owner of the store set me up for success.  I was able to find the bulb and replace it.  The water is looking better already, and I just replaced the bulb on Sunday!  Since I was at the koi pond store anyway, a few water hyacinths followed me home. They are a nice addition to the pond.