Sudden Unknown Issue?

[LadyLouise]

Hi this is my first time posting here, so please forgive me if its a bit long.
I have had my four blind cave tetras for four years, and after years of them being in a 10 gallon, I was finally allowed to upgrade. I put them in an 36 gallon with an aquaclear 50.
After a while, just having the four in such a huge tank was sad, so today I added six gold barbs. I floated the bag for about 10 minutes, and put them in. However, I didnt know i only had one net, and it wouldnt fit in the bag, so I had to just dump the water in with them . Not my finest moment, but the fish at that place are very healthy, and it is owned by an aquatic enthusiast, so they are well taken care of.
In less than 20 minutes, as I was checking on them, I noticed one of my Blind Cave Tetras was swimming on an angle, and spasming every 15-30ish seconds. The tank has been set up for a month, he was fine before, the filter and heater are working and the water looks fine. He continued spasming and swimming sideways all night, but seemed to do it less when the light was off. It's been 5 hours since the initial incidents. He's still the only one acting up, and I'm worried. I added a little bit more water conditioner, in case there was an issue with the water in the bag. Other than that, I don't know what to do it looks like shock, but the temperature and quality didn't change and none of the other 9 fish in there are acting different! Please help

Tl;Dr: 4 tetras for almost 5 years, got new tank, waited, bought more fish. Added water too. One tetra started swimming sideways and spasming.

 

[Far_King]

Firstly I'm going to address the way I would introduce fish to a tank.
 
Lights off to reduce stress.
Place the bag in the tank to receive some of the heat from the surrounding water.
After ten-fifteen minutes, open the bag and mix in some of the water from your tank with the water the new fish are already in.
Leave for a good half an hour then mix some more in.  Again leave for half an hour or so, then you can place a net over a bucket and tip the bag into the net, separating the fish from the water and immediately release them into the tank.   Some people may advocate a long period of time.  I would recommend using a quarantine tank rather than adding fish directly to a display tank.
 
It's possible one of the Barbs nipped your tetra.  It possible it was shock but I think that is less likely than some sort of direct physical injury.
The only time I've seen a fish do this before was when a neon I had was hit by a large pleco  when he decided to unexpectedly thrash around.  That was the last time I kept neons with fish that size.
I would keep a very close eye on the behaviour of the barbs.

 

[LadyLouise]

Thanks all of his fins look normal, and the barbs are about half the size of the tetras and scared of them. Thanks for the advice om adding new fish. Im about to go check on them, wish me luck.

 

[Far_King]

You're welcome.  I can only hazard a guess really.  Gold barbs are the least likely barbs to nip.  
 
I hope your fish recovers.