IF you have used copper, then the copper could have been absorbed into anything including the seals of the tank. Assuming there was copper somewhere in the tank, then it would leach back out into the water and that could kill your inverts. So you are right to be concerned. If you haven't used copper then you are probably fine. If you aren't sure then you'd probably want to pick up a copper test kit just so you can watch the level to be certain.