I hope one of you can help.
I have a garden pond that needs some work doing on it. In the pond are koi carp, goldfish and those white and orange ones (can't remember their name

).
Each year there is a bit of hanky panky and I get new fish (I don't know which ones are doing what with whom), but so far new goldfish have turned fully orange and the orange/white ones have turned orange white.
I need to do some structural work on the pond which will require partial or total draining and disturbance of habitat. The question is when is the best time to do this? Any other guidance?
Thanks in advance.
SAB