PAR meters measure the actual available light, usable by the plants, where ever they are used. If you have 50 mms of PAR, it doesn't matter where it came from or how it got there, it is still 50 mms of PAR, equally good for plants.
Carpeting plants need CO2 much more than they need high light. You need to get the CO2 ppm in the water up near the point where the fish are very unhappy, then even with low light you can grow carpeting plants.
I plan on raising it. I may even get the TEK 4 bulb fixture - that way, no matter what I do in my future, I don't need to worry about purchasing an additional fixture (could do a reef tank with this fixture). I will also be able to utilize the front and back bulb placement for the widest light spread over the tank. I will be hanging the light so I can do any distance above the tank necessary.
Can a light be too bright when directly above tank, but when PAR is reduced by raising it a foot more-or-less, the light quality is diffused such that it is no longer good for plant growth particularly carpeting?
I guess part of what I am asking is all PAR equal and thus you can't have a light that is "too bright" when close and "not good enough" when suspended higher from tank, at least when talking about Hagen GLO and the TEK fixtures?
The Hagen GLO fixture looks pretty good to me - at least it is wide enough for light from the back of the bulbs to get around the bulb to the tank. You could raise that 2 bulb fixture as much as a foot and still get good light, but with much better uniformity. It is going to be more light than is easy to manage, if you don't raise it to at least 6-8 inches above the top of the tank. Having good CO2 allows you to raise it even higher, and still get good healthy growing plants.
Now I am considering going back to unplanted with the 20 gallon tank (long story, none of it bad, but mostly to do with sig other and her plans for the tank) and upgrading by replacing it in the living room with a 40 gallon breeder. I have a thread active in the lighting forum again and i was looking for your input, on a 40 breeder which is VERY wide (16 inches?) is a single 2xT5HO fixture by Hagen GLO good enough for lighting? I will figure out a way to suspend it probably through bent conduit tubing (ceiling fixtures won't fly) - I know that 2 T5HO gives high light but I am just concerned about the spread throughout the tank. Are you familiar with this brand and their reflector? If I am better served spending my money on a different fixture (Tek 4 x 39W, 2 bulbs only?) that will provide better spread or quality of light than I can save up...
My pressurized CO2 is a shipment of fittings away from being ready, minus the reactor/diffuser!
Moss should grow very well with that lighting. CO2 isn't required, but CO2 always helps plants to grow faster and more healthy. At that light level, you probably don't need to do any fertilizing, assuming you have a normal number of fish in the tank. It wouldn't hurt to dose Flourish, which is a trace element mix, since fish poop may be deficient in trace elements.
Thanks for your assistance on that thread. With that lighting (coralife 2 x T5NO on 20g high tank - 16 inches from substrate) do you think I could do a moss "carpet" like I have seen done with plastic pieces, without CO2? I am willing to do CO2 if needed, or EI or other fert method if that would help too.