Korky Flapper https://amzn.to/2MyJmJU
It's easier than you might think to fix a leaking toilet. Almost always the leak is from a bad flapper. It takes less than 5 minutes and you don't need any special tools to do it. Don't worry about the water in the tank being dirty. It should be clean. It is certainly not the same as the water in the bowl even though they are close, the water in the tank hasn't been contaminated with sewage.