Two possible causes. First...the fish are bigger this year than they were last, adding to the organic load in the pond. I estimate your pond is less than 700 gallons or so. In all honesty, you are overstocked with 4 koi and some goldies. Koi need a minimum of 300 gallons each and that is pushing it.

Second problem...did you get a new UV bulb this year? Typically they only last for a year before they lose a lot of strength and cannot perform well. change that bulb out and you will probably see an immediate difference.

Also...you say you have lots of sludge? where is the sludge? Are there rocks in the bottom of the pond trapping it? A bottom drain should carry all the sludge to the filter where it can be removed from the system. There should be no sludge in a pond with a bottom drain. Could you provide a little more information about how your pond and filter are set up?

Sorry that's my other pond that I just keep plants in. The pond with the fish is 8x8x3. I didn't get a new UV bulb, but I did just buy one so I'm hoping that helps. The sludge is in the biofalls. I just got done sucking it all out. I neglected to clean the biofalls in spring and the sludge was deep & disgusting. I know i probably messed with the natural good bacteria, but I had no choice, I had to clean it up.

Bottom Drain is connected to skimmer and skimmer to bio falls.

the sludge in your filter has nothing to do with the good bacteria...it is rotting fish poop and uneaten food and needs to be cleaned out on a regular basis, not just once inthe spring. Seriously....cleaning the sludge out once a month is not even often enough. I clean all my filters at least once a week. The beneficial bacteria lives on the filter media, so do not wash it hard or with chlorinated water.

Adult koi need about 1000 gallons per fish, but if you are very regimented with your cleaning tasks and have a really decent sized filter, you can get away with less.

First of all, you need to get all of this "sludge," out of the pond, and you have to do it often. Also, change the water. Not all of it, but a portion. The more you do it, the less you have to do at a time.

Secondly, your filter. The easiest way to cheat on the "gallons per fish," rule, is to oversize your filter. A large filter will increase the volume of your pond, and a bigger pump will turn over water faster. The more water you have, the bigger the bioload the pond can sustain. The small filters sold in places like Petsmart aren't really the way to go in this situation. You're better off making your own or buying from a pond supply store.

Third, algae. Algae is pretty much the most persistent form of life on the planet, if you ask me, and throughout the life of your pond, you will battle with it. .Algae feeds on the nutrients in your water. The more nutrients, the more algae. Your nutrients are coming in the form of fish food and waste. Koi are big, dirty fish, and they put a lot of "nutrients," into the water. A UV filter is not going to do anything for a pond that's way overstocked. Algae is not ALL bad, however. Having small amounts of it will help keep nutrient levels down. In a healthy system, your aquatic plants will absorb most nutrients, keeping the water clear, and out competing algae. Algae will always be around in some form, however, and that's okay. It's the huge blooms of it that you're trying to avoid.

Fourth, the fish. Personally, I love koi - they are beautiful, endearing fish, and they are a joy to keep. But they are demanding. They need lots of room to grow, and a small system just isn't the place. You're better off with comets, which stay much smaller than koi, but also come in a good range of colors. If I were you I would try to give some of the fish new homes.

Basically, what's going to keep your pond clear is hard work.

On a seperate note, the whole bioball system is rather questionable, and many people find that over a period of time they begin to contribute large amounts of nitrate. They are designed to excel at breaking down ammonia, but overtime waste gets trapped in them and thats where the problem comes from. You might want to research that.

I wish you the best of luck. =)