Nitrate at 50 ppm is too high. You need to keep it no higher than 20 ppm, and as low as possible is best. Like ammonia and nitrite, it is toxic to fish, but it just takes longer and the detriment is more "hidden." Not suggesting that is the issue here, but it doesn't help. [Is this all from the aquarium itself, or do you have nitrate in the source (tap) water? Test that on its own if you haven't already.] Live plants would also help. And how frequent and what volume are water changes?
The GH at 100 I assume is ppm (not degrees)? This is equal to 5 dGH. And not high enough for livebearers to be healthy. However, your tetras will prefer this, so before thinking about hardening the water, you might want to reconsider the fish.
The pH at 7.4 is not too high for livebearers, but with the GH and KH low the pH is likely to slowly lower. Fine for tetras, not for livebearers.
I agree with Demeter on the temp. What tetra species are these?
Clearly you have soft water, so soft water species is be the way to go, and there are so many. Almost any fish from South America and SE Asia (with very few exceptions) will be right at home. Livebearers sadly will struggle and weaken.