You need to do water changes. Try to get ammonia and nitrite under .25 ppm
. Ideally it should be 0. I assume you know about the cycling process? You appear to be almost done. Rummies aren't good for cycles. That is why one died. You should return the last and get other fish. Rummies really need a tank that has been cycled for at least a month. More is better. What test kit are you using?
Welcome to AA.
Lol just kidding we aren't that boring! I mean, that is the advice I wanted to give but we don't talk in that short sentences!