Seems with those dimensions that you would have way more than 5 litres of water. My pond is about 15 x 19 and has a small falls and holds about 2800-2900 gallons (about 11,000 litres - sheesh, that sounds like a lot). 5 litres is 1.4 gallons.
I also have gold fish and one koi. And as fish are wont to do, I get babies each year. Mostly the fish eat the babies so very few make it to adulthood. And as I have a very prolific black looking gold fish, they are quite often black, very difficult to see in my pond until they get at least 6" long.
Soon we will be shutting the pond down for the winter. Just put in some deicers, run a pump to circulate water from the deep end to the shallow end, and hope for the best. Spring is always full of surprises when I see one or two fish come out of dormancy from the cave they overwinter in. Pretty soon all 20 came out this spring - and amazingly a bit larger than the previous fall. Quite amazing that they grow, even in dormancy. What is your weather like. I see 'uk' so assuming you are in Britain-Scotland- Ireland somewhere?
