How do you identify if your carbon needs to be replaced? It's hard to notice the slow change in water color in the tank as the carbon is exhausted; here's an easy way to test it using two white buckets. Step 1: Fill the first white bucket with freshly mixed saltwater. Step 2: Fill the other bucket to the same level with water from your tank. Step 3: Compare the bucket of tank water with the freshly mixed saltwater. If it looks yellow in comparison, it's time to change out your carbon. We took the opportunity to do a PAR test on the two water samples while we had them handy and found that the dirty water lowered PAR from about 2100 to 1625, nearly a 500 par difference, and the water wasn't even that dirty! If you're going to spend the big bucks on a high performance light system make sure to use carbon to keep from squandering that extra par. Connect with us on Facebook: http://www.facebook.com/BulkReefSupply Check out our pics on Instagram: http://instagram.com/bulkreefsupply