I bought retrofit kits for my lighting and built my own canopies to save money. My lights are 5" to 6" above the water, but the reflectors still get water spots on them over time. The main culprit I have found is the oxygen (?) bubbles that the plants put off during the day ... sometimes the bubbles come to the top of the water and sit there, then they pop and often spray a little water droplet up to my reflectors or lights. It's a slow process, but I do feel the need to clean the reflectors every couple of weeks -- which I don't like.

So, I'm thinking about putting either acrylic or glass into the canopy just below the reflectors. I'm sure this will cut down on the available light, but how much? I have 4 wpg (2 x 96w PC for a 50G tank) right now, so for me to cut the light down by 25% would be no big deal. Do any of you know whether acrylic or glass would reduce light less? Also, if I do put glass or acrylic below the light, should I lower the lights closer to the water?

Or perhaps, I should I just clean the reflectors once in a while and leave everything as is ...

By the way, I have light fans, so over-heating will not be a problem.

Acrylic doesn't work well as a shield on top of the tank. Acrylic absorbs water, which causes it to warp badly, since only the side next to the water gets water to absorb, causing that side to swell. And, the water spots you now get on the reflectors would be on the shield if you use one, so you would be cleaning that shield even more often than you clean the reflectors - it would be closer to the water. I prefer to do without the shield.