Fish tank water level

Question
Hi,
This is a rather unsual question for you. I have had many a fish tank in the past, my largest being a 600 litre tank. All ran very well. I have now moved into a small apartment in a very old building which was built in the 1880's and since renovated. We have a very large outdoor pond which only has two large goldfish in it. so i bought a small 2ft tank to buy some baby comets etc to grow before releasing in the pond....the strangest thing is happening....I filled the tank up with water from the pond as the pH level is perfect and getting the nutrients and bacteria from the pond is ideal. within 6 hours the water level of my tank has dropped by 2/3rds !! so i topped it up before i went to bed and in the morning, theres barely enough water left in the tank for the fish to swim....so i'm topping the tank up from almost empty to full every 6-8hrs....and there is absolutely no water leaks as there is no water to be seen anywhere around the tank or the stand. im using a fluval 204 filter (which was from one of my old larger tanks), but also, no leaks or anything around that either. could it be because of the house? dry air? any ideas would be helpful.
Cheers,
James

Answer
Hi James,
Thank you for your letter. There has to be an explanation, because according to the laws of physics, the water cannot just disappear. Maybe it is going out the filter, maybe a cat is drinking it, maybe a ghost is drinking it . . . oh, that doesn't fit in with the law of physics, either.
When you find out what it is, please tell me too!
I wouldn't blame it on the age of the building unless the ghost theory is correct.
Nori