If you are talking about your problem in your other post,as I wrote there,I don't think the open jaw has anything to do with how many fish you have in your tank.If it not a disease,it will not affect your other fish.If one dies,make sure you take it out of your tank promtly.I for one also like to use live to cycle.After all,damsels don't cost too much.Just keep an eye on the fish.If there are no signs of disease,things should be fine.hth