Well a lot depends on what you want in the tank as far as fish. A 40 I believe is 12 inches deep and that is something I avoid now a days in my tanks I prefer 18 to 24 inche deep tanks (but I tend to have larger fish). I do have a 45 which I set up with a 280, heater, and I threw a extra skilter 400 I had on it (it does not skim the 400's are worthless except as a water mover in my mind. In that tank I have a maroon clown, snowflake eel, coral hogfish, and a cleaner wrasse. That tank gave me nothing but problems and killed a lot of nice fish for months then somehow it settled in and has been fine for months and has great algae growth going on the live rock I bought for the eel to live under. This tank proved a point to me tho as I used copper in it during my problems and now it has 2 large turbo snails and algae growth (so the copper seems to have gone on its own with just water changes after the med period. Fill me in a bit on what you want to do with the tank.