A 40 breeder tank is about 16 inches high. If you have 3 inches of substrate, and the bulbs are about an inch above the top of the tank, they will be about 14 inches from the substrate. A Coralife 2 bulb T5NO light should give you about 40 mms of PAR at that distance. If you use two of them spread about 8 inches or so apart, you should get pretty uniform 40 mms PAR over the whole substrate. Unfortunately it is hard to find any Coralife T5NO lights now. You could do without CO2 with that light level, but Excel would help the plants a lot.
The feline situation has convinced me to re-look at the whole picture here and after reviewing many of your comments in the past again, now I am back to thinking about a T5NO, 2 bulb wide-spread setup in a canopy. then I don't need to mess with anything between the lights and water and there will be defense against the cat going in which is win win.
I have a 2 bulb T5NO already on my 20 gallon High (16 inch height, 12 inches wide) and it certainly doesn't seem too bright to me although I have no PAR meter. It is a coralife which has mixed reflector reviews. I am struggling to convince myself that the same number of bulbs, but spread out 8 inches from one another in a bigger fixture, will provide good lighting for growth. Does that bring me to the equivalent of 1 T5NO for most substrate areas?
I have never tested the use of "egg crate" versus light intensity. Intuitively I think it will reduce the PAR about 30-50%, even directly under the light, and more further away from the center of the light, but that is largely a guess. (A spot directly under the light gets light from a pretty long length of the bulbs, but the "egg crate" stops the light from much of that length, which should reduce the PAR directly under the bulb.) I doubt that there is much, if any difference between 10" and 11" wide fixtures.
[FONT=Arial]I can get a 2 x T5HO custom Catalina fixture in either 10” or 11” wide (which must be used for their 3/4/6 bulb units) for a comparable price to the Hagen GLO etc fixtures and less than 50% of the price of getting a 4 bulb fixture (TEK etc) and not using 2 of the bulbs. This seems like a great way to increase the spread over an 18” wide 40-breeder tank.
A recent change; I will be using egg crate over the tank to prevent feline intrusion. I understand that egg crate re-directs stray light with an increase in PAR directly beneath the bulbs yet a decrease in PAR as horizontal distance from the bulb grows. Do you think 2 bulbs in either 10” or 11” fixture, will be able to be raised to an “optimal” height such that I had the relatively even low-to-moderate light you recommend w/ CO2 dosing/EI? See any reason to use 10” vs 11”?
I have various photos of tanks I have had, and setups I have had, but my 3 tanks now are all in a state of need for a lot of loving care. I get preoccupied with my projects and neglect them. And, in a few months I hope to be moving, where I may not be able to have any tanks, which dampens my current enthusiasm.
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.