Using carbon in your reef will help improve water clarity, which allows better light penetration for your corals to utilize. I've been using GAC in a reactor for years, and never had any problems.  Perhaps because I follow the rule of thumb of using only 1/2 a cup of a carbon per 50g of water.  This video discusses how to use it actively, as well as a brief update. Carbon can be purchased from the shop area of my site: https://melevsreef.com/reefshop/dry-goods/media