I agree sounds like a cycle, but I would check the Nh or ammonia level in the tank. If it is a new tank and the ammonia levels are high ;"I would try a small water change and add some more live rock to the system" to compensate for the nitrifiying bacteria levels. If it is not a new system are you using carbon or a skimmer? A skimmer is a deff best bet for the dissolved organics you might be smelling plus if you are keeping a mantis you are more prone to higher nitrate levels bc of the silversides ,krill, live foods etc. I keep diff macroalgaes in my mantis systems for nutrient export also. I would also use a High grade of carbon in the system this will aid in clarity and any oxidant that might find it's way into the tank. Hope that helps.