thanks for the reply. in my case i want low light, and my tank dimensions are 13x13in surface area, so i currently have a 23w daylight in a 8.5in dome and the bulb does NOT stick out... in the mean time, based on your previous experiments, which chart should i be using?
When there was big disagreement about how much PAR one of those bulbs would give, there were 3-4 of us making measurements, and making various errors in doing so. Plus, we found that a bigger CFL bulb that would stick out of the dome reflector would give significantly less PAR, and we were mostly using 8.5 inch domes, not 10 inch ones. With the 10 inch dome the bulb doesn't stick out, as I recall, but with the 8.5 inch one it does. I haven't revisited that subject since then, but I have planned to do so sometime.
You would be providing a very limited lighting spectrum if you use just the actinic bulbs. Plants do use that light, but they need more than that, as far as spectrum is concerned. And, most people find a planted tank with just actinic lights looks very weird, spooky, and lacking in colors.
Any 4 bulb T5HO light is likely to be too much light for a 20 gallon tank. But, the Aquaticlife lights also have good reflectors, so with all 4 bulbs on you should have about 150 PAR at the substrate. There is no way to get enough CO2 in the water to fill the plant's needs to grow at the rate that much light demands. That is certain to lead to big algae problems. Even with 2 bulbs you will have to be very careful to get enough CO2, enough oxygen in the water, and good enough maintenance to be able to avoid algae problems.