When I purchased my pond heater August in August 1990 at the National Show I realised that I needed to put some type of cover over the pond to conserve the heat.
For the first few years I laid the large bubble wrap over two thirds of the pond. It is possible to obtain these sheets in 2.4 metre widths so this reduces the number of joins.

This works quite well as long as you remember to wet it well after installation and it will usually not blow away when there are all but the strongest winds.
The main disadvantage is that the fish have a habit of jumping on top of it, and then cannot get off. There is also the disadvantage that with the bubble wrap laying on such a large part of the surface area then gases cannot escape to the surface.

This photo shows the cover in position over the pond.
In 1996 when these photos were taken the cover was a translucent tarpaulin as used on trucks to cover their loads. This year I have used a different cover similar to those supplied for polytunnels, and will have photos of this later when I have had time to check its performance.

This photo show the end closest to the filter, and has access to the pond. The total length of the pond is 18.5ft and the length of the cover is 12ft. The remainder of the pond is covered with bubble wrap lying on the surface. In this area there are trees growing over the pond so a complete cover was not possible.

Click on any image for a larger jpeg image of 50kb.

This is a view inside the cover. The framework is made up using 20mm and 25mm electrician's plastic conduit, and the general construction can be seen in this photo. It will be seen that some of the black uprights are not in their correct positions, but these have now been correctly held in place with screws.
Last year the frame was dismantled for Summer storage, and this October it was quickly erected in just a few hours.

The two photos above show the framework in position ready for the cover to be put in place. The ladder is in position only to help with the fitting of the cover, and the positioning of the bubble wrap which is held in position by wooded battens which have been treated with G4.

Photos of problems with the cover have now been added.
Check out another cover built by a koi-keeper.