you can use super glue GEL (important that it is the gel), but just take a small piece of rock out of your tank and let it dry for an hour or so then put a good amount on the rock and let the glue start to dry a little bit then push the mushroom into the glue gently but with a little tiny bit of...
yeah im agreeing with everyone else get the remora its a great skimmer for small tanks, and easy to use also the best performance for your money, you can find some good deals on used ones on ebay and various other places.