First of all...welcome to cigar pass, fill out your profiel, and post an introduction, this great bunch of guys would like to know a little about you!
Second, you should definitly be unsig distilled water. I don't know if your sponge would be "ruined" from using bottled water, but you might want to flush it pretty well with distilled once or twice to remove any impurities. that would also likely remove any propylene Glycol(sp?) still in it, so you would need to add some of that...the lesson? throw away your sponge, and buy some beads (65%) from Viper (heartfeltindustries.com) You will not regret it.
If you are having to add water more than once a week, there is likely something wrong with your system. Was the humi seasoned properly? Are you relying on a humidity reading that is low to tell you to add water? If so, has your hygrometer been properly calibrated?
Whew! I know, a lot of questions...but there are a lot of factors and variables that go into making a new humidor work well for you.
Good luck, and I hope this helps.