Wow, interesting. We did not fill before removing the black mesh cover, but Mother Nature did it for us - we had a very big rain and with a mesh cover, it goes right through, so it filled to right below the return. I pulled the cover off and added bleach - stir a bit with the leaf net (no leaves, just a tiny bit of silt) but got that bleach all around in there. We did not leave the cover on for long though due to me being a worry wort and thinking I might get algae.