Where Did The Water Go?

In our basement not far from my computer desk, we have a 150 gallon tank. This tank is home to Peewee, the turtle, an unnamed plecostomus and a crayfish that likes to hang out under the filtration system. Since Peewee is still quite small we have the tank set up to be about 15 gallons of water with gravel land should he want to walk about. Last night I was doing some computer stuff and noticed the water was getting low. I usually add about 5-10 gallons a week so I already had a 5 gallon bucket filled and setting out. I got the bucket and added it. The water still looked low. So I added another 5 gallons without my normal 24 hour wait to get the chlorine out.
This morning, I went to say good morning to Peewee. And there was no water. It was like a peninsula of gravel with small amounts of water on the side where Peewee has scraped the gravel back over time. Poor Mr Plecostumus had found one such pocket of water and was hanging out there. Where did all the water go? I knew that I had added 10 gallons the night before. And now there was hardly anything. A quick glance through the tank revealed the culprit, the air tube. My son had been playing with it yesterday afternoon and inadvertently created a siphon. All afternoon and evening the water tricked out of the tank onto the carpeted floor. I figure probably 20+ gallons. Doh. I have added 4 gallons ad will gradually add more as the day goes on until it is back to normal. The guy at the pet store recommended water changes but I don't think this is what he had in mind.
Bad news: I have a mess to clean up and carpet to dry out.
Good news: Son learned the effects of a siphon and won't be playing with the tube anytime soon.


Here is Peewee and the Plecostomus after the I added the 4 gallons.