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Here is the whole situation:

55g Tank:
5 Serpae Tetras
10 Cardinal Tetras (now 4)
2 Golden Gouramis
No live plants (some algae at back of tank)

The fish were introduced in that order after 1-1.5 weeks each.

Everything was going fine until I woke up this morning and 5 of the cardinal tetras were dead (either suctioned to the filter or floating around). I counted 5 cardinals left swimming around, in the time in took me to remove the dead fish, do water tests, and feed the other fish one more cardinal was dead suctioned to the filter.

My first thought was "it was the serpae's" because serpaes can be bullies to other fish. But oddly enough the serpaes and cardinals have been together for around 3 weeks and not a single incident has occurred. The 2 golden gouramis are the new guys, I added them 4 days ago and it seems weird that they would just now decide to murder. I cant put my finger on whether it was one of the other types of fish

So I tested my water:
pH:7
Ammonia:0
Nitrite:0
Nitrate:25

Although it would be optimal to keep the nitrate at 0 in the tank, without live plants its a little more difficult. But around this level of nitrates has been in that tank since my zebra danios cycled it, and since all my permanent fish moved in. So it still stumps me as to what has changed to make them all the sudden die.

Observing the surviving cardinals doesn't answer anything either, they all have very good color, very minor fin nipping damage, and are swimming fine, they are more timid, however, this may be because they aren't a proper school anymore. Anyway, if anyone has any questions, solutions or ideas, please tell me

Do you use a liquid test kit or strips?

I'm pretty sure zeketaz is asking about the tests you use since some can be more accurate than others. Another issue is how old the test kit is, older ones can sometimes not be reliable.

Since the gouramis are the new ones I'm also slightly suspicious of them, rather than outright killing they could just be the new presence which is putting extra stress on the cardinals.

When you're feeding do you watch closely to make sure everyone is getting enough food? Recently we'd lost a few of the humpheads in the tank since I hadn't realized the kribs had been beating them to the food, now I watch extra close to make sure that everyone is getting enough food at feeding time.

Since you don't have live plants do you at least have a good number of fake ones for everyone to feel safe and hidden in if they wish? We redid our tank to include a lot more hiding spots so that the remaining humphead has plenty of options for getting out of the way when the Kribs make a dart at him.

I would not try to get nitrates to 0, especially not in a planted tank, that's just asking for BGA to move in IME. Nitrates in the 10-20 range are generally good with most fish, especially if you're doing regular water changes, it's sudden spikes that can cause more issues. If you can some java fern tied to some pieces of wood is easy and doesn't need any extra light. Look for a piece with little fernlets coming off the leaves which you can then tie onto the wood as well to have more plant cover in a few weeks.

Only other things I could think to check is that there is enough aeration of the tank, and that the heaters are working properly. If any of the fish seem to be gasping at the surface more than usual I'd increase surface agitation.


^_^

The test kit is liquid and is a few years old but it matched the results from the pet store where I got my refund. The tank does have a lot of fake plants to hide in and 3 wood/rock decor to hide in and behind. The tank has agitation from the filter and a bubble maker and the tank is at 79 degrees at the moment. I believe everyone get enough food, I usually put some at the surface to distract the others and then some in the filter stream to go down to the cardinals. The interesting thing is the single living cardinal, is still alive, its surprising. It seems like they might have just gotten intimidated by the new gouramis, maybe the gouramis took their hiding spots?

That is possible. If you have a small tank you might look at getting a few more tetras to fill the school back up. Keep them in the small tank for a bit before introducing them into the larger tank.

^_^