New tank, small amount of Nitrites, lots of fish poop!?

I recently have set up a brand new fish tank, it came with a filter&I have bought a heater,some live plants&some ornamentsThe aquatic shop I bought them from advised me to put Water conditioner&Stress Zyme into the tank(which I have done)&I left it for a week.I took a sample of the water back to the shop for testing&results were...Ammonia 0, Nitrites 0.5. PH - Fine (whatever that is).So the shop keeper was a little confused as to why there were Nitrites in the tank when I had have no fish whatsoever.He advised me to go for 4 platies to start with(my tank is 35l) which I have done today,II acclimatised them before placing in tank with temperature &water quality&they seem to be doing fine,however the shopkeeper told me not to feed them all week&come back next week with more water.My problem now is that there is a plant in the tank which the platies seem to love&they wont stop eating it,I have took the plant out&now there is lots of poop which I am worried might kill them What do I do?