The full travel of the key is 1/4 turn and the lid should "pop" open after just 1/8 of a turn. You're already able to turn the key 1/4 turn so something might be amiss with the hard plastic catch under the lid. I'm at work and don't have access to pics but I'll try and describe what's under the lid.
The key turns a "cone" shaped piece of hard plastic. There are two grooves(one on each side) cut into the base of the cone piece. When the lid is closed the cone spreads two pieces of straight wire(spring latch) which then snap into the grooves at the base of the cone, holding the lid shut. When the key is turned the cone spins so that the grooves are no longer facing the spring latch and the lid should pop open.
So thoughts as to why you're able to turn the key but the lid won't pop open? The spring latch may have rusted up over time and become quite rough. It could act like sand paper against the cone piece and worn some shallow grooves onto the "non-groove" surface. If this is the case then light prying while turning the key may be enough to free the lid. Not enough force to bend the lid, mind you, just enough to add some force.
If the cone piece is somehow seperated itself from the key tumbler and is not turning with the key( I don't see how but anythings possible,right?) then the bent butter knife route may be your best option. If that's the case then I can post some pics of the latch under the tank so you can get an idea of what you're pokin' at
