cardinal tetras are extremely delicate fish! Anything from a change between the PH level in the store tank to when you add them to your tank with different PH level - could kill off cardinals. Never mind issues with chemistry/water parameters.
I bought 5 Cardinals, and lost 3 in the first weekend I put them in the tank. I was lucky two survived, and they are now part of a neon tetra community.
Cardinal tetra should not be considered 'easy' beginner fish.
As for cucumber feedings - My otos loved the cucumber - the cories had no problems with them in the tank. Maybe my Bettas were suffering from dual problems - too much mulm in the water - my planted tank is definitely a work in progress. The new filter could have stressed them out too.
I don't know if I want to feed more cucumber to the tank. I've heard that otos love zuccini - so that's the next veggie on the list...
I'll avoid giving more cucumber (at least) until I resolve the excess plant detritus within that tank. I do believe I have more siphoning to do, and an assessment of what plants are better suited vs. not suited for my light source.