Before we go any further.
1) Plecs and swordtails are tropical species and do best in tropical temperatures, 24C and above. Goldfish can survive this temperature, but prefer below 22C. What temp is your tank?
2) What are the ammonia/NitrIte levels? Anything over 0 is toxic and can leave long term damage. If you don't know, either go buy a liquid test kit or get your local fish shop to test it, just make sure they test it in front of you and explain it as they go.
3) How old is this set up? Is it a cycled and mature tank?
4) Setting up the other tank could either be a good idea, or a far worse idea. If it's an infection, it could spread to the other fish, making the Q tank a good idea. However, since it's a much smaller space than the 55g, water quality would be far harder to maintain as there is only a fifth of the water volume compared to the 55. This means that there is less water for the pollution to dissolve into, making it more toxic in a shorter amount of time. You would also need to do ideally daily water changes as the tank wouldn't be cycled but assuming your 55g is cycled, you could get round this using some filter media from your other filter. A non-cycled tank will only make things worse.
5) Plecs are armoured catfish and as such, are intolerant to salt, adding salt could stress them out to no end. I've seen it used successfully, but I've also seen what happens when it goes wrong.
If you can give us more information such as water stats and temp, we can be of even more help.
