Diagnosis
Ink got into the air vent channels of the tank during filling, which causes a siphon effect and overflow. Or, the rubber seal on the tank's cap isn't seated perfectly.
How to Fix It
Immediately power off the printer. Remove the leaking ink tank. Wipe the outside of the tank and the inside of the printer bay with paper towels. Check the small air vent hole on the top of the tank—make sure it's clear and no ink is pooled in the channel around it. Re-insert the tank until it clicks firmly.
If That Didn't Work
The tank itself might be cracked or the cap seal might be damaged. Inspect the tank for hairline cracks, especially near the handle. The caps are supposed to be replaced occasionally; if yours is old or stiff, it might not be sealing.
Heads Up
When filling, always use the bottle with the spout and fill slowly to the max line. Never fill above it. Overfilling is the number one cause of this. If ink has gotten deep into the printer mechanics, it can cause serious damage and may require professional cleaning.