I wouldnt reccomend coconut fiber as they could eat it and choke or get bunged up in their digestive tracks.
IF you are worried about excess nutrients in it, do what I do. The teabag method:
Take a towel and put your soil in it and wrap it like a "tea bag"
Grab it with at the top with the towel ends so you have a "sack" of your dirt.
Take it to the tub and rinse it under warm water, massage the dirt and break it up.
The water will be red or yellow colored if there is excess salts.
Rinse until the water running out is brown or black.
You now have "organic" soil
...ahem, all soil is organic...
Like I have heard on here before. Plants need these nutrients to grow well, too much is a bad thing, and your cham eating dirt (which is ok, cham's can eat some dirt) that has high concentrations of these salts may make him sick.