Gudgeon (can't spell it) turn the bottom of ponds over. Some tench are helpful? but if it is too bad fish alone won't get rid of it. Ponds that get too mirky need to have a decent filter sytem that will remove all the rubbish. Our pond is crystal clear all year round. We have a big filter system which drags up all the rubbish from the bottom and gets rid of it all. It may be a costly out lay (depending onn the pond size) but it is far easier in the long run. Instead of cleaning the pond you are cleaning the external filter instead - easy - job done! Pics of our pond are in the gallery?
Be careful with coldwater fish because some of them need a licence now. You can check on the PFK site for a list of fish that you need a licence for. Just giving you a heads up in case! Get1956- didn't you already have an overcrowding problem in your pond