i also went for the large columbo black substrate, because Ps3Steveo uses and his tanks are amazing, and the reviews looked good. Very happy with it my previously melting Blyxa Japonica really took off again. I took all my fish out, inc 11 neon tetras, and 2 red cherry shrimp at the time.
I put the colombo florabase on top of the old black gravel+sand as it wasn't a very thick layer, left it a few hours, regularly rinsing the filter floss, until the cloudiness came down a lot, then put my fish + shrimp back in as I don't have another tank, all of them survived, but maybe I was lucky! I did do 2 extra water changes for that week also.