beginners questions

hi, i have just got my first tropical fish aquarium. set it up two
weeks before adding any fish and took the water sample to shop, the
advised it was ok - a little high on the nitrite side but nothing to
worry about too much.

i have a small tank so started with 3 guppys and 4 tetras. all was
well for 3 days and then 1 one of the tetras died. i have done my own
tests for nitrite and amonia which seem ok. have done water changes of
about 5l from a 35l tank. what am i doing wrong?

I was told to feed the fish only 3 times aweek and as such so far they
have only had two feeds - they seem to eat it really quickly when i do
feed them - am i not feeding them enough perhaps? (they only get
literally a couple of flakes).

any thoughts ?

thanks