Just read through the octopus care and journal and photos forums. There are tons of pictures of octopuses people on here have kept/are keeping along with details about their octopuses activities. You can get to know what to expect out of different species and what to expect with keeping one.
A 75 gallon tank is suitable, sizewise, for most of the octopuses you will find in the trade except a Vulgaris (too small) and Mercatoris (too large). After reviewing the care articles you might also read through the tank forums threads for an idea of what you will need to do to your setup and which (I assume you have multiple tanks from your initial post) of your tanks might be the easiest to convert.