As Ed said, check salinity, if the full 20 gal went in at one time - could be a low salt condition, but you can't correct it all at once. You need to slowly add in a galon or so of higher salt content (1.028-1.030 specific gravity) each half day or even once a day, so things have time to get used to the changed condition before changing again.
Taylor is on the right track if you had just been adding water as normal over the past few weeks to get to the full 20 galons of water since the switch mix-up. Heater being set high (how high?) could also be an issue.