Posted: 08/07/2016 at 19:24
You 'could' wait for it to fill with rain ecokid but you might be waiting a surprisingly long time. I personally would fill with tap water as chlorine dissipates after a week or two anyway. Unless of course you have a water butt full of rain water.
When I did my pond I had an algae bloom after a couple of weeks but once those nutrients get depleted the algae will die off, especially when you introduce your higher plants like lilies and irises