By filtration I mean does the Fluval have like a sponge filter or some type of media that can host bacteria inside the filter assembly? Your salt level is fine. I wonder if there is chlorine or chloramine in the water. If you have any seachem prime (or other similar water conditioner laying around), you should go ahead and dose that. Maybe some other toxin from the rocks? Other than those things, the only thing I'm left with is ammonia/nitrite as cause of death. I just don't understand why you're ammonia hasn't gone down with everything I now know about your system after you added the microbacter. Unless you have stray current going through your tank that is killing your bacteria and fish...but you probably would've noticed a tinge when sticking your hand in the water if so. This is a real mystery to me. Dose some Prime and then dose from a different bottle of bacteria (biospira, fritz's, etc.). Then check your ammonia/nitrite/nitrate levels again after a couple days. If ammonia and nitrite aren't gone after that, something really weird is happening. Tons of die off in the rocks somehow? I don't know.
