Your confusing me a bit....your title says windshield washer tank but then you say coolant? And on the windshield washer tank there is only one hose at the bottom, plus the plug for the pump and the low fluid plug just above that.....
If you indeed are working on the windshield washer reservoir, there should be a small bolt towards the bottom. Follow the neck down with your hand and it should be 3/4 of the way down, either an 8 or 10 mm socket.