Hi NexusIcon and Girlfriend!
Welcome to TFK! Please don't be discouraged by this. We've all been led astray and we've had moments just like the release of your Tetras I'm afraid. Very disheartening.
From my experience... I'd like to share a few things.
I've always kept 1 Gourami to a tank. Even the gentler Pearls will chase and harass their own kind. I had 2 in a 75 gallon... that's a 300L approx.... and one harassed the other to death. Also be aware... sometimes it is impossible to tell if you have M/F's in some species until they get much bigger... When you see a tank of lovely colorful Dwarf Gourami... those are ALL Males... the Females are boring drab brown and your LFS rarely bring in both as they don't sell as well. You unknowingly bring home 2... and you have 2 fighting fish. Grrrr.
I want to go over "release" as well so that you are successful with your next purchase.
But first... How long has your tank been set up? Has the tank gone thru a full Cycle... this process can take 6-8 weeks. We strongly advise you spend the money on a liquid testing kit... they run about $30 and you will know right away when something is amiss rather than seeing issues at night and not being able to do anything until the next evening. When in doubt... do a water change.
Water changes... make sure you have a good quality Water Conditioner. Most of us use and love "Prime". It is a bit more expensive but highly concentrated... one cap does 50 Gallons. And it neutralizes Ammonia, Nitrite and Nitrates!
Did you lay the bag of fish on the surface to equalize the temperatures over time? I open the bag and clip it at the surface and each 5 minutes add some of my Tank's water. Over a half hour I will have doubled the contents of water in the bag thus equalizing the temperatures and mixing any differences in PH. When the time is up I net the fish into the tank. It is never good to pour the store's water into your tank.
Eventually, we advise a 2nd tank for quarantine but we understand not everyone has the room or the luxury to do this. Buy and add small quantities of fish each trip, 3-4 max... never bring home 13-15 at one time! (done that!
... too much bio load and many will die)
Just remember it is the things we do quickly that can have the most negative effects. A big PH change, and water change with the wrong temp water... that type of thing.
There is a question/answer diagnosis page on the Forum as a sticky somewhere in here.... lol.... If you walk yourself thru this you will see areas/questions that you can't answer right now... those are the areas you need to read up on! as your understanding of these areas WILL lead you to success!!
Found it... post number 1... it really will help you understand where you are: Diagnosis Form - Read this before you post.
It helps us help you too when we know "where you are at" in the process with a new tank.
Anyway... sorry for the long post! We're glad you joined us and are here to help!!!