still need more info tank size, filters,food u feed,times u feed,where u get ur water from (well,tap,bottle etc). Have u gotten these bichirs from the same lfs(maybe a problem with there supplier) if they have a small amount of bichir and they interbreed to much this may just be natural, how often do u clean the tank. u should take the water from your tank to ur lfs and have them test, then have them test there water so u can see the difference. For some reason i am thinking to much salt in ur water