Congrats, QTOFFER!
I posted this question quite a while ago. The suggestions I got were to empty the tank (I waited until the betta died
) and rinse with a bleach solution. Rinse the tank very well, let water sit in the tank and add lots of dechlorinator, empty and rinse again. I also read that letting the tank sit out in the sun will help remove bleach residue too.
There are still some stains on my silicone, but it has lightened. I don't think I used enough bleach, because I was too afraid of not getting it all out! The stains do lighten in time, but they are still somewhat noticable.
Another way, if you're handy with
DIY projects, is to take the stained silicone out, and reseal the tank. There are a few sticky posts or articles that tell how to reseal a tank.