[tang daddy]

Above poster has good points. I'd like to elaborate if I can....

The T5 you run are going to be better than the stock Bc29 cpl lights.

That being said if the moonlights are too bright then you don't really have to run them, 4 t5 over an 16" or 18" cube is plenty light so the next time you decide to change the bulbs make sure to invest in some KZ or ATI bulbs as these are by far the chosen brand for most reefers. Also the spectrum can be played with, if it was me I would run 2 blue 1 14k and 1 purple. If your fixture comes with separate switches so you can turn on 2 bulbs at a time, you can run the 2 blues for 8-12 hrs and the white purple for 4-6 hrs.

Addressing the open top, when the lights come on you could have a fan blow across the tank to keep the bulbs and your cube cooler. This will prolong the light fixture and bulbs aswell. You can build a screen top by getting a few simple things. Lowes or Home Depot sells aluminum window kits that are safe to use over the tank. I pair this up with plastic chicken wire from Canadian tire.
If you are unsure about building one go to you tube and check out the videos on building screen tops for reef tank.

You may have to double up the mesh to make it smaller as fire fish can fit through standard 1/4" mesh.

For heaters I would suggest a titanium heater with external control, they aren't very expensive about $30 but gives you piece of mind because they won't break like glass, fully submersible and won't catch on fire.

Hope this has been helpfull.