I have a 20L established tank with 1M & 1F apisto plus some dithers. They have been together since they were juveniles and behave more like brother and sister than a couple. I'd like to introduce a second female but the fish that are sold are always juveniles and considerably smaller than the one I have. Does anyone who has done this have any insight into how the older female will react to a new smaller female? There are a lot of caves, wood and plants.