Disappearing Water Mystery

This is one mystery that had me baffled - I wondered if this (or anything like it) had happened to anyone else

A couple of years ago, I had a large fish tank - containing 2 goldfish and one loach (I had had them for about 1 year - in the same tank - in the same position in the house)

The tank held about 30 litres of water .... maybe more 2ft x1ft x 16in high

I got up one morning and noticed that that the tank was about half empty and the goldfish were floating on there sides gasping (not looking too good) .....

The tank had been fine the night before (and full) - so I Instantly groaned - thinking that the desk the tank was set upon and the carpet would be saturated.

I headed into the kitchen to get a heap of towels - thinking that I had a massive mop up operation ahead of me - however - there was not even a damp patch - the desk was dry, the carpet was dry ?????

I moved the tank into the kitchen and moved the desk - nothing - about 15 litres of water had gone - I havn't got a cat ( or any other pet that may have drunk the water) - It is just friggin weird.

The fish lived for about another 5 days but I could not revive them.

I refilled the tank - and the water level remained constant thereafter -

Any Ideas