Honestly, i think it may be the guppies that are killing your other fish. Although they are peaceful fish, they carry diseases. Guppies are very resilient when it comes to diseases, so they carry them without any repercussions. When you add other fish to the tank, that are not guppies, they contract the diseases that the guppies carry. I started my 20g tank with a couple guppies and they were fine. I was in the same situation you are now, without the plants. The guppies are since gone, but from time to time i still get fin rot back into my tank. I think its stuck on my ornaments, as i have already changed the rocks and all of the water.