I always like to start from a known situation when setting up a new tank. I find there are all kinds of questions when starting and I like to have a definite idea of what I have to cut down the possible surprises.
The easy way to know what you have in the tank is to throughly clean it . A simple bleach water filling, soak and draining resets all the things that might be in the tank.
I just feel better about not having the unknown suddenly show up, whether it is live things, oil or pesticide, I want it gone!