You definetly need check valves and a bubble counter. check valves keep the water from flowing back into the co2 tank. Bubble counter lets you monitor how much co2 is going into the aquarium. Others things you might consider but you don't need is a co2 regulator with timer. Unless you want to always manualying turn your tank off every night, i would get these. It's also good to get extra diffusers since they tend to break rather easy and for cleaning, since you can take one out and soak it in hydrogen peroxide to remove the algae. For co2 fillups, sports stores won't fill up tanks over 5lbs, but any specialty beer store will for usually around $8. My 10 gallon co2 tank usually lasts 4 months.