09-15-2007, 08:14 AM
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Yes it can effect the output and spectrums. I run ever tank I have as an open top. It also helps reduce the temps inside the tank. The glass will filter out UV rays and reflect up to 50% of the light.
I'll try to to help as best I can from your limited write ups.
Implying this is a planted tank first, fish tank second.
Your spectrum "K" should be less than 8,500K or 8.5K. K is the reference to Kelvin, a unit of measure for heat. They found a way to measure the lights wavelengths in regards to that of the sun, 6.5K is supposed to replicate the suns natural output midafternoon. Higher K factors lead to fluorescent colors as you are removing the "white" and "red" spectrums that we associate with looking natural. 20K bulbs are the ones people use like black lights to make the colors really pop. 6.5K bulbs will really make your plants grow but may look to dim or yellow. 8.5k will add a touch of brightness to the tank. Anything under 6 will look really dismal and yellow. Most hydroponics shops will recommend 6.5k as it seems to produce the best growth.
On to lighting itself.
You don't mention the size of your tank. If is rather large and or small with a heavy emphasis on plants, you will want metal halide or sodium vapor lights. They will add lots of usable light to the tank. The wave lengths pulse, this simulates the sunlight and can stimulate growth. T12 fluoro, like most aquarium bulbs and shop lights, can work well for smaller tanks. You can also buy HO and VHO set ups. A basic T12 is about 20-40w, a VHO tends to run the same bulb at 90-130w. HO is high output and VHO is very high output. You'll need special ballasts to run them. There is also T8 and T5. The benefits from these are that you can fit more of them in the same space as T12. They are basically smaller diameters. Then you get into power compacts, basically a T5 bulb bent backwards into a thin "U"shape so it's comparable to 2 T5 bulbs. Why choose one or the other? Cost is the most significant factor to me. I always prefer metal halide for situations where it counts, fluoro for smaller tanks. Why would I pick T12, T8, T5, or power compacts? Basically for whatever is a good deal and in my budget at the time. VHO set ups do not generally come in a pre-built fixture and are typicallly retro-fit only. T8's are not common in the trade, but are at HDepot. They typically come in 6.5k so work really well on planted tanks. That's a serious consideration as the cost is only about 25% of that of a specialty shop. T5 are the newest emerging champion, you can order them anyway you want. If you want fluoro lighting in a nice fixture you can find good T5 setups but they may be expensive. Power compacts are nice but have been in steady decline for about a year. The T5 setups are easier and cheaper to produce.