What makes some fish species more difficult then others? The reason why I ask is I have a 125 gallon reef tank that has 3 chromis 2 clowns. I had a bellus angel that went into qt and was fine for 4 weeks then after 3 weeks in the display got sick and died. I also had a sailfin that went into qt for 4 weeks did great healthy transferred to the display and after 5 days got sick and died. Any help would be great. Thanks.