Wow, what a mix. Well, a few things I can see as potential problems...
The fish are a bit mis matched, and territory for those fish in a 20 long will be an issue if you want to keep peace. It's good that there is a school of the serpae tetras, but just to be aware, they tend to be nasty, nippy little things. In a 55 gallon I still would not mix serpae with swordtails because I'd be concerned about agression levels.
Rope fish also can be quite evil when it comes to aggression levels. The one my ex bought yrs ago and dumped into my 55 gallon dwarf cichlid tank managed to eat my expensive cichlids within months. I came home from work one day and it had a blue ram in it's mouth. The ram was too big to swallow, but that rope fish carried it around in his mouth for almost 2 days, unwilling to give up his prize. I was in the process of finding it a different home after having chewed out my ex for putting it in there in the first place.
How long has this tank been set up? When you do gravel vacs, do you get under the plates of the undergravel and suck out the muck? I'm not a big fan of undergravel filters unless you are really into live plants, and have a fully planted tank. They cause issues long term if you don't clean under the plates often enough and don't have enough plants to feed on that "muck"/nutrient levels they are designed to provide.
Please be aware, nobody was attacking your LFS. All we were stating is that for us to help you, we need those numbers. Your LFS may be able to help you when they do the tests because they have the numbers in front of them, and maybe don't feel the need to give you those numbers. If you wish to learn to help yourself, we can teach you, but that means always having those numbers handy. "Just fine" tells us nothing that we can work with. One person's "just fine" might be considered deadly to another, and gives no indication of what else we would need to look for.
I would highly suggest working on seperating your remaining swordtail(s) out of the current tank. There is a strong possibility that what happened was a matter of aggression bewteen fish. Also, I would not advise keeping 1 male and 1 female swordtail togther. Each male will need at least 2 females to chase. With just a pair, the male will likely chase the female until he exhausts her to death with trying to breed. This is typical of livebearing fish.
I look forward to seeing the numbers for your water params. After that, I may be able to lend some further insight.