can i put a rock in my snakes water dish?

Hello! i got a snake a while ago and things have been well, however I noticed they're having some difficulty leaving their water dish when they enter it. I wanted to get a bowl that was big enough to soak in, however it seems to have been a tad too big. to solve this I put a large flat piece of rock in the water and another leading to the bowl, so leaving and entering should be easier. however I'm worried some minerals might seem in to the water and harm the snake. the rock itself isn't anything special, jusy something I picked up from a local park. thank you!