Dead Guppies

Hi, I recently got some guppies and a swordtail, a little over a week ago, to put in with my other fish. My guppy had babies, developed a curved spine, and died. A swordtail has a discolored patch and I have noticed missing scales on a few of the other swordtails. Then, my other female guppy had a discolored mark and these red-spots as shown in picture, and has now died. Any idea on what's going on and what I can do? I suspect Columnaris. I posted on another forum and others didn't seem to think anything was wrong. However, now she's dead so I'm pretty sure something is going on. Please, help!

 

I'm planning to treat it with Fish Amoxy and Kanaplex. Also, I have already added Metronidazole to the tanks and I plan to continue with that. Any thoughts?

 

I think it's been running for about a year.
Going off the dimensions it should be a 20 gallon.
I don't know exactly how many. I've got alot in there.

 

I also have a smaller one. I think it is around 3.5 gallons. That one has a guppy and a few baby swordtails. It's been running for probably a few months by now.