Question
I have a 2,500 gal commercially constructed pond and have experienced problems with string alge and some water clarity problems, at times.  I use an upflow design combination, filter and waterfall and clean it twice a year.  It is appropriately sized for the pond .I also have a prefilter in the the skimmer.
I would also be interested in hearing about the construction of your own filter.

Answer
Hi Richard,
Sorry I am a liitle late answering your question but I have had PC problems,
For clarity you maybe should clean your filter more than twice a year,when you clean it DON'T clean it with tap water clean it with POND WATER,also apart from adding addatives to your water or pulling out the string algae there is nothing more you can really do.
If you want to see my filter go to my website at http://koi-z-are-us.20m.com and click on the filter page.
Hope this as been of some help,
George,