since you are in the planning stage, another thing you may want to do is take a step back and look at it from a system design viewpoint. what needs to happen? source water handling... RO/DI, water storage, mixing, transport to /from the tank (for water changes and top off).. granted this may seem like overkill for a 40g tank, but the system design concept is scalable, so when you find that 125G tank (or larger) on craigslist a year or 2 from now you will feel like a genius rockstar