Aquariums: NSFW because of dead kardinal tetra, advice needed

Hi everyone,
I (23F) am fairly new at having my own sweet water aquarium. I have corydora, sherry barbel, 2 other algae eating fish and got 5 platys and 10 kardinal tetras today.
I checked water levels before getting them at the aquarium store and all were good. As soon as the tetra were in my aquarium they were already in a veey stressed state.
Now 3 hours later, i came to see how they are and 7 of the 10 are laying dead between my plants. 3 of them still alive but almost drained of any colour and still very stressed.
Is this normal for tetras? Did i miss something important here? Understandably i am very upset about this, if i put my mind on taking care of an animal it is my intention of doing so the best i can..

• tank holds around 97L, 80x40x40. Filled with plants and real wood if that matters

TDLR: added 10 tetras today to my tank along with 6 platys and the tetras died within 3/4 hours.