The amount of light doesn't have to exactly one amount or another. And indeed, different kinds of bulbs have diff outputs and for fluorescents the match between bulb and ballast makes a diff too -- not to mention that the light output ratings are usually gauged to human vision and not plant photosynthesis (which are not the same ranges). But watts per gallon a ready-at-hand substitute for all those other measures we don't have or don't have the time or industry to develop. And it turns out that watts per gallon works pretty darn well. If you're going to use fluorescents with high quality reflectors, then 1.5-2 watts per gallon for slow grow and 2-3 watts for fast grow. If you add CO2 and ferts *and* keep on maintnence and are careful to keep nutrients in balance with each other, then you could try 4 or 5 watts per gallon. But fast tanks can much more easily race out of control. The faster the tank, the more sensitive they can be and the farther out of whack they go in a short time. More than 2 watts per gallon and you seriously need to consider adding CO2. Of course you can add CO2 at the lower lighting levels too and the plants will just love it. But don't add a lot of light without adding CO2 -- there's no law against it and surely some folks do quite well as exceptions, but you are much more likely to have problems with lots of light and no extra CO2. For slow grow, you could use a pair of 13Ws over a 12 or 15g, a pair of 36Ws over a 40g, four 55Ws over a 90g, pair of 96Ws over a 74 or 90g. These aren't all the same watts per gallon but they fall in or near the range. And remember, all electric light bulb give off less light the longer they are used. So if you're little on the high side of 2 watts per gallon (like the 4x55 on the 90 example above) just be content that you will use the bulbs much longer before having to replace them? If you try and figure things closer than that, then your wasting a lot of time (unless you like doing math and shopping more than gardening). Your own gardening & maintenance style is going to have as much or more impact than you are likely to realise by trying to fine tune between, say, 2.1 and 2.5 watts per gallon. Some plants don't even like a whole lotta like; some are very demanding about it. Some of those you might enoy growing and some not. So start out simple, with the basics. You can always get more complicated whenever that looks like it might be fun. Some of us try equipment just to see how it works, but it's not recommended as a practice for all hobbyists. Some build rooms with hundreds of tanks, but it's not a requirment for enjoying the hobby for most folks. You can try to research a lot on specific bulbs, research the specific plants you have in mind, do calculations based on dessred changes in biomass, factor in filtration capacities, turbidity, angles of refraction. . .literally drive yourself nuts -- Or you can just follow the rules of thumb that seem to work well and spend you time enjoying your fish and plants.comparison of light bulbs Based on this, Daniel Larsson concluded:
Bad for red pigment development: Philips965, Philips950, Philips 940, WarmWhite, AiMDayHiGlow, Hagen Marine Glo, Hagen Flora Glo, Hagen AquaGlo, Hagen SunGlohttp://www.naturacquario.net/lux/fluorescenti.htm http://aqua.passion.online.fr/pages/douce/articles/eclairage.htmlAverage: Sylvania Aquastar(!), SylvaniaGroLux, Aim Biolux, Arcadia Tropical, Arcadia Freshwater, Hagen PowerGlo, Hagen LifeGlo
Good for red pigment development: Philips 930, Philips Aquarelle, Philips 840, Philips 830 (best?), Philips 827, Arcadia Marine White