Something is wiping my tank! :(

I am having some serious trouble with my 55g tropical community right now. My brain is a little fried today but I'll do my best to give all information needed. The tank as of last week was stocked with 3 dwarf gouramis, 4 mollies, 2 platies, 1 swordtail, 11 neon tetras, 5 black neon tetras, and 5 fancy guppies. I do a 25% water change weekly with tap water conditioned with Tetra AquaSafe Plus (Prime on order from F&S).

So here's the problem. In the last week I've lost 2 fancy guppies, 1 black neon, and (up to) 8 of the neons, as well as a twin bar swordtail today. Sadly, not only does the problem not seem to be letting up but I can't identify any symptoms or signs of what could be wrong. All of the fish who have died, as well as the ones still alive, were and are acting perfectly normal. One of the neons was swimming around rather sparatically and doing barrel rolls which I found quite odd. Every other fish in the tank seemed fine though. They are all fully colored and active as well as eating normally. There are no ick spots, external parasites, swollen eyes, red gills or bloating. As far as I can tell, nothing is wrong, but I keep waking up to dead fish.

I took the dead swordtail and 3 of the neons I could find to my LFS today and one of the employees dissected the swordtail and found about a 1/2 inch red parasite inside the stomach of the fish but nothing in any of the neons. The employees, as well as myself, don't believe that an internal parasite could be wiping my fish so quickly. Regardless, that is all we had to go on so I've treated the tank with a dose of Copper Safe to try and rid the fish of any parasites but I do not know how else to progress. Any information would be greatly appreciated as I'd really like to not lose my whole tank :'(

Water params are as follows:

Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 20 ppm
pH: 7.6
Temp: 79

I do not yet have a test kit for kh or gh but I have one in the mail. Nothing has been added to the tank recently that should have caused this. Please help!

All fish are either from my local Petco or PetSmart because there isn't a good true LFS anywhere near me. I picked up the swordtail about 3 months ago and 5 of the neons last week. All fish were bought between these times.

First of all: yes, their resident aquatics expert was examining the fish. I wouldn't call it a true dissection but didn't go into full detail in OP. He was handling the fish in the bag I brought it in and the stomach ruptured. There was nothing unusual except for that parasite. If you Google search "roundworm in fish" you get exactly what we found. It was about 1/2 inch and still alive.

I was afraid maybe it was bad stock but for so many to die so close together really concerns me. The guppies could have died from being used as cycling fish (I know) and the neons were on sale so maybe already weak or stressed? The swordtail could have just coincidentally died at the same time from the worm, but that seems like a stretch.

The only thing that could have contaminated the tank is my girlfriend using Windex to clean the front panel. She only did it once and it was well over a week ago now. Closer to two actually. Needless to say that won't be happening again.

Hopefully the Copper Safe will do something. The fish that are still alive have been in the tank for a while and all seem very healthy so maybe the worst has passed. Keep the comments coming and I'll also update the thread with any developments.

All that may be true but it seems strange to be losing the stock so quickly and so close together. Especially after the cycle has completed and the levels are now normal. None of the fish even showed any signs of stress during the cycle. I know the conditions weren't ideal but I only let the ammonia spike so high because nothing else was working.

Everything is fine as of waking up today. All fish seem fine and no new casualties. Thanks for the input. Ill keep everything updated.