One fact is that 5 cardinal tetras is about all the fish I would ever recommend for your tank Tess. You may well find resources that suggest you could get by with more fish. My own experience is that staying on the light side with stockings pays off in the long run. No matter the unexpected circumstance, my lightly stocked tanks seem to do well and the ones where I push right up to the stocking limits do not do as well. It is a personal preference at this point but is one where I will always encourage conservative decisions.
