How long have you had tap water in there? If it is just for a little while IMO you just need to drain it out good.
Forget about the water pump until the easy stuff doesn't work.
You can buy distilled water at the grocery store in gallon jugs. It will be labeled distilled water ( NOT SPRING WATER that is the same as tap. There are minerals in there that start corroding all of the coolant passages in the motor and radiator and will clog stuff up and make the car over heat if left long enough). Buy a few they are pretty cheap
So after you drain all of that tap water out just unscrew that bleeder screw on the t-stat.
Fill with the same mix of coolant and DISTILLED WATER this time
Fill until water squirts out of that t-stat bolt hole in a streaming like manner.
Replace the bolt and finish filling the rad up.
Start the car up and watch for overheating and leaks.
There are other ways to do this but try this way first