Other considerations for a perfect water gardening
Reed bed system. (See Vegetable Filter or Mini Reed Bed Systems) Create a cascade of containers. Three is generally enough - one above the other, all containing gravel in which fast growing marginal plants are planted. Water is pumped up from the pond, pours in at the top of the top container, flows from one container to another and eventually back into the pond, after having passed through the root systems of the plants.
This system not only clarifies the water, but also removes masses of toxins and pollutants that most other filter systems dont come anywhere near to removing. Norfolk reed and the different species that come under the common name of water cress are particularly good. In my experience Villarsia (Nymphoides peltata) works well too.
CURES FOR
Scum or oil
Drag underlay or cardboard (even newspaper) across the pond as you gradually pull it up out of the water. The scum sticks to the surface as it is drawn out. The cardboard, paper or underlay can then be disposed of. Slow but sure. Larger pools can be dealt with by using a boom created from a floating sausage of straw, linen, socks or wool. In an emergency "necessity is the mother of invention."
Red water
Common in birdbaths and small bodies of water, red algae (Rhodophyceae class), perhaps Batrachospermum, seems in my experience to be a pioneer and merely heralds the arrival of the more proliferate green algae.
If it was very red water and this was a detective novel, one could immediately suspect blood, but since it is not, suspect paint or food dye and then blood especially if there seems to be a body floating in the pool.
Brown water
This is usually a sign of mud stirring fish activity or turbulence. Rain can also batter particles into the pool or stream. Koi carp digging around in Lily baskets are the bane of many water gardeners' lives. Tench feed on the bottom and can be a problem if the silt on the bottom is very fine.
CURE: Heavy pebbles on the surface of the baskets. A 'Netlon' tube protecting the plant to the surface, or fish free zone in the pond reserved exclusively for plants.
Alternatively, get rid of the tench and the Koi.
Sometimes brown water is simply caused by using the wrong type of soil in the bottom of a wildlife or natural style pool. Some particles of silt and humous in soil simply do not settle out.
If you are going to put soil in the bottom of a pool it should be inert subsoil or as in the pool under construction by the British Trust of Conservation Volunteers do, just use heavy Holme sand around the top edge.
An English Country Garden by Brian Aughton and Theresa Potter at the 2002 Tatton Flower Show. Some times the stone that is used stains the water, particularly after heavy rain.
Clear brown water
.like tea can be the result of the leaching of minerals from the surrounding rocks and soil or peat bogs. Tannins can also diffuse out of leaves like oak to produce a similar staining effect.
Black water
If you push a stick into a pond containing water like this, bubbles rise to the surface. This is due to the decay of organic matter. The bacteria involved in this process are not the desirable ones and are producing poisonous gas and toxins. There will be trees in the vicinity shedding their leaves into the water. I expect some of them contain toxins too.
The organic matter in the bottom must be removed a soon as possible. Do not disturb it too much as it will poison the water even more. There is probably little enough oxygen in there anyway so a partial water change will help as an emergency measure, until it is feasible to do a total clean out.
Try to net the pond before the fall in Autumn to prevent this becoming a perennial problem.
Milky water
It is highly likely that there is a dead fish or animal decomposing in the pond. Get it out and change the water.