have the fixture go on top and the arcadia t5 12% is perfect for that. This depends on the screen -- if the screen is a fine mesh its still going to need to go inside the tank - the screen is going to block 30% of the rays - so we can determine that when you get the tank -- fine mesh like a screen door vs a wide hole screen - get the 40 gallon for now -- find a dresser if you have one and use that or look at some used furniture stores for a dresser w/ drawers to fit - I bought one for Blazes tank for $40 works great and I use the drawers for his supplies food extra bulb etc -- just get the measurements of the tank and go from there --