We recently got gifted a 20 long aquarium (30x12x12) that used to have all sorts of tropical fish in it. Not sure how old the tank is, but some of the stuff that came with it is from the 60s. Fish were in it as recently as 2014. Looks as if it has been stored a few years.

Is there any special way I should go about cleaning this thing, to avoid disease transfer and leftover toxins (if any)?