Caryl's suggested method works well enough for me (I don't like the taste of fishwater to be honest). In smaller tanks though where it's a bit too fiddly to get the whole tube submerged, an easy enough alternative, with a bit of pre-preparation, is to get the lid off the tank, place bucket nearby - walk to nearest tap, fill hose and block both ends with thumbs...walk back to tank and insert one end of hose in tank, other end in bucket - remove thumbs (figuratively speaking that is) - hey presto - flowing water with fresh breath