Substrate change

Question
Hi Karen!  I have a 55g tank with about 25 small-sized fish.  Pearl gourami's, swordtails, tetras, golden gourami's(?) and one bristlenose catfish.  It's also well planted.  I would like to change the substrate though, from a horrible marble (I think it's toxic too, isn't it) to a sandy medium.  Everyone says something different e.g. 'all in one go with removing fish, plants and decor' and one says 'do it in two go's without removing anything.  Half the substrate, wait two weeks and then the other half with disturbing as little as possible'.  This seems more logical.  What do you think?  The more I read up, the more clueless I feel.

Answer
Hi there!
I think you're on the right track about the changing the substrate over by sections. I've changed substrates over many times before. All you really need to do is visually divide up the substrate (from the front of the tank) in sections, the easiest is just long rows. I've done about a 1/3 of the substrate at a time, every week to every two weeks. If you have a well established tank and a good filter, you could probably do half per week/two weeks just fine. We just want to be careful and not wipeout the bacterial colonies too drastically. But there is no need to remove all the fish or decor. Just switch decor and plants to another area and re-set it back once you have put in one section of substrate. Everything should be just fine done this way.

Best of luck!!
Karen~