Thinking about adding LED moonlights to your light fixture? I recommend doing so with double sided adhesive tape made specifically for "Picture & Frame Hanging". This type of adhesive tape holds strong on many surfaces including your light fixture and removes cleanly without leaving a sticky residue!
Shown in this photo is the Neptune Systems Apex Lunar Simulator LED modules mounted to the back side of a ATI Sunpower T5HO light fixture. This placement allows it to be completely hidden from view when looking directly at the aquarium, while still being able to illuminate the aquarium for night time enjoyment.