don't buy a tang to solve this problem. Answer some questions first. how long has your tank been set up? What are the water parameters (nitrite, ammonia, nitrate, PH, ALK, ect). If your set up is new then this is normal for a tank finishing it's cycle. What kind/size of clean up crew do you have( snails, hermits). If your set up is new and it appears to be then one major suggestion would be to remove the CC and replace it with a deep sand bed about 4" deep. It will be easier to do it now and the tank will look better and the DSB will reduce nitrates which cause unwanted algea growth.