Hi,
Is your pond filtered, uv'd, aerated, heated?
What temp is the pond water? The last few nights have been very cold which could have put the fish off feeding. Have you tested the pond water for ammonia and nitrite recently? If the blanket weed is concerning you can you remove it manually? If you use a chemical blanket weed killer it can open a whole can of worms, I think it's better to remove it manually and live with a small amount if you can. As for suspended solids/algae it depends on your filter system, and what you want it to achieve. Crystal clear water requires good mechanical filtration.
Chris.

Welcome to the forum, it would be helpfull if you could give more details on your pond. Such as do you have a filter and how big is it, how old is the pond and how many fish are there in it.

Do you have a test kit, are the ammonia and nitrite readings ok? Also are the fish flashing at all?

Sorry for all the questions, oh and take out the uneaten food.

It sounds as though they probably have flukes but there's cleverer people on this board who may be able to say more.

I don't think that your questions are connected.
Question No.1- I have a bit of the silt-like stuff floating especially near the plants. It makes the pond look quite dirty and yet when I pull it up with the plant (usually one of my oxygenators like canadian pondweed) it seems to disappear, and the plant then gets thrown back in. I don't know if yours is the same. Also in my shallow end of the pond I find that it settles and when you try to pull it out it all just floats into little specks again and can't be removed. I have found that it does disappear in time or settles onto the bottom of the pond. There is nothing wrong with that as long as it doesn't get too thick on the bottom. I now use my net to try to get the silt-like stuff away from the plants. As I pull it out I realise that it is changing into the slimy algae. When it does this it can be removed easily. The stringy blanketweed around the liner is often used by the fish for food. All my tadpoles are living in it at the moment so I don't take it out unless I check it all first, but it is easy to remove. Take out any that you don't want. Eventually most of it should disappear anyway.
As to your second question about the fish being at the bottom and not eating much. Mine are the same at the moment. I think it has a lot to do with the weather and the temperature of the water. Last week they were up all week and in the shallows but this week when it has got a bit cooler they are back to staying in the deepend and not eating. I don't think you have anything to worry about. Another thought which might make you worry is that a preditor has been by, maybe a heron has flown on over and scared them. If so you are lucky to still have your fish but it is something that we all worry about.
Alison

Here is a link that might be useful: Visit Alisons website

Good point Alison has made there, they could just be spooked by a predator. How big are the koi though? big ones wouldn't be spooked for long i dont think and at this time of year would still come for food. I wouldn't have thought the temperature would have changed too much in a 5ft deep pond. Here in suffolk I'm having to hold back on the food as they are very hungry in a 2 ft deep pond. I still suspect a parasite problem but it all depends on the answers to our questions, if it's a new pond with new fish it's probably that.