I've been using selftanner lately, will my tan wash off in a chlorinated pool?

Actually it will turn you a pale shade of green, so watch out.

do you think a thick layer of sunscreen would prevent that?

Maybe, you’d have to experiment.

second, how long does it take for it to change color? like do you notice it right away or does it take like 6 hours? (I really have no way of expirementing until we go to the waterpark, and I do not want to be green then, but from your knowledge)

I believe it depends on combinations of skin type, the specific self-tan chemistry, the strength of the chlorination of the water, temperature, and length of exposure. No way I could safely answer your question with 100% accuracy without all those details.

Long exposure to water, with or without chlorine, will fade your sunless tan. An expert explained to me what happens: your outer skin cells will start to soak up water, and that dilutes the effect of the sunless tan. The longer you are in the water, the worse it will be. Using sunless tanners regularly over a period of time will saturate the skin better and the sunless tan will hold up longer in water, but it will still be affected. Some people report that applying a waterproof sunscreen before they get in the water prevents their sunless tan from fading as quickly. But no sunless tan can stand up to extreme conditions such as a day spent in a wet suit.

Source (It doesn’t say anything about turning green, FYI.)

Some are formulated to be worn in chlorinated pools. Look on the website of the company that makes your product.

there is not a website available for St. Moritz self-tanner, but since its supposed to be near identical to St. Tropez, and on its website it says avoid water may bleach tan.