I wanted a little more formal look without it costing too much. So I capped the top edge of the pond with 2x8's on the flat, ran 1"x1" strips of wood along the inner and outer edges, and then set ceramic tiles in between them. They are not glued down and grouted since they wouldn't withstand the extreme cold of our winters and would crack and let go. This way, I can remove them, clean out the crud underneath, and just put them back. Things need a little freshening up but it's holding up well after 8 or 9 years.
No problem Alex. Water retains its heat very well compared to air. So even though the air temperature may drop 10 or 15C from day to night, the water, although cooling some, will only drop a degree or 2 overnight for the volume of water in my pond. The pond contains ~1200G and the larger the volume of water, the more stable the temperature will be. I have a 100G Little Giant stock trough outside as well, but it will drop ~5C overnight. If the water temperature change is very gradual, the fish aren't affected adversely. About 5 or 6 years ago, I was a little remiss in getting my fish out of the pond in the fall, leaving them out until November 1st. There had even been a couple of frosts before that. The day I pulled them out the air temperature was 1C and the water temperature ~3C or 4C. I had sunfish, goldfish, some Uruguayan Gymnos and Australoheros out there along with a 12" pleco. When I pulled them out they not only had survived the cold ... they never looked better in terms of coloration and condition. Lately I've been putting more 'tropical' species out, so, I monitor the temperature more closely and aim more for mid October as the end of pond season. The lowest I feel comfortably subjecting my fish to, is low to mid 60sF (~16-18C), with Uruguayans down to 10-15C. Hope that helps clarify things.
