Duckweed

My small garden pond has been very neglected and is full of duck weed.  I have got as much as possible off with a sieve attached to a bean pole but as the water level is so low I have found all the oxygenating plants are full of it as well.. Presumably the best thing would be to replace all these, so would this time of year be the best time to do it?  I can't use chemicals even if I wanted to as my dog treats is as her personal water bowl!  

Isit it ok to top up with tap water, I haven't any rain water left in my water butt.

thanks,

jayTee