Upsize - This is the biggest one and every day I look at this 16g cube and kick myself. I've had FW tanks for years so I know more water = usually easier to manage. Plus, now that I know more about the saltwater side, there's fish and coral I want that would be impractical in this tank
Better Light - I bought a 50 dollar light off Amazon. It worked okay, but you get what you pay for.
Better Aquascape - My Aquascape is fine as it is, but I should of did more with smaller rocks versus 4 -5 big pieces. Really limits what I can have as the main rockwork of the tank is in high flow/high light. My inexperience makes me worried about putting SPS there since I am still working on the tank's stability.