when i pulled an air pump(for airstones) off of the 75g, i forgot to take the airline tubing out, while those were laying on the floor, it created a siphon and over the night sucked out like 20g of water. When we filled it back up, the water surface was being electrocuted by the canister filter(we think because it was running dry for so long), but after plugging and unplugging it many, many, many times it seems to have been fixed.
The moral of this story is: NEVER LEAVE A TUBE IN YOUR TANK, YUO'LL BE SORRY!