If you are running your fluval with carbon, stop using it and fill the compartment with ceramic rings. Carbon has been linked to a possible cause of HITH. I once had a pair of jags that developed HITH and I came across the article that said carbon dust could be a cause. I stopped using carbon and the HITH cleared up. I've never used carbon again and never had a problem. The carbon cause for HITH is only a theory but I beleive it because it worked for me.