Well I am going to assume you are shopping at Petco. First thing is that java fern has to be planted on a rock or wood. Next those two bamboos on the left will die being a non-aquatic plant. When they start to look sickly just pull them out and put them in some dirt. you will be fine leaving the plants you have in when you add the new plants. Word of advice is to just wait. Let you current plants mature, and you will have plenty to work with. Amazon swords get very large, and need an incredible amount of space, which will take half of your 20 H. I would advise against getting one. There are several dwarf species of sword that will do fine in a 20. That java fern you have will also get quite large, and will also give you plenty of daughter plants which can be pulled off, and rooted onto a piece of wood.