If I do a 50% water change and add carbon to my filter, about how long do you think it will take for all meds to be gone? I have a 20G tank. And was treating with Maracyn2.

I don't know.
Here's a good page on what carbon removes but I don't see M2 on the list.
http://www.wernersponds.com/Activated%20Ca...for%20Ponds.htm
isn't M2 erythromycin?

I looked it up. Maracyn is Erythromycin and Maracyn-2 is Minocycline. I'm not a chemistry major so this means nothing to me.
I also have an emporer 330 bio wheel. So it's pretty good filtration for a 20G.

Just wondering cause i want to start with a different med and do not want to mix.

Actually I might just start with a dip, but then I wanted to use some tank water and don't want meds in it. Do you think it's ok to just use a gallon of bottled water instead of tank water for the dip? I don't want to stress my fish any more than he is with a huge water difference.

I was told that with a filter that filters 10x the water per hour that 24 hours should have all meds removed.

What kinda dip?

Those would work if the pH, etc isn't too different from their usual water.

Bottled water will work if it is at the same pH and the same temp as the tank water. And yes, carbon does remove Maracyn 2.