I too have this peoblem. The water looks brownish in colour. I did a 95% water change last week cause of this problem but its back again after just 7 days.

The pond is in full sun is about 3500l has 2 10000l filters to clear it and has about 26 goldfish in it of varying sizes. It also has shelves on either side for pond plants and a couple of pond lillies. There has been no rain fir about a week either.

Any ideas?

The murkiness in your water is suspended algae. Depending on the mix of algae, the nutrient levels, the amount of sunlight that hits the water and the time of year, the water may look deep green, greenish-brown or brownish-black.

The problem of controlling suspended algae in fish ponds is probably raised more often than any other question. I am sure it will not surprise you to learn that a comprehensive answer would take up many pages. Let me offer some straightforward solutions.

Algae are plants, and they require three things to grow: water, light and nutrients (phosphorus, in particular). Because the water is a given, let us consider the other two factors.