Out of my several planted tanks with community fishes, I have one which for some reason has an algae problem. It is quite a small tank by my standard, 36G, with a total of 180 W of compact F L. The break up of K ratings would be 108 W 8650 K 72 W 4500 K. The photo period is 10 hours.

The plants in the tank are varieties of crypts, 2 patches of dwarf crypts and 2 of taller ones. Some water-sprite, an amazon 'ocellot', and a patch of bacopa, a patch of ammania gracelis, and a patch of ludwiga repens completes the flora. The substrate is the same as all my tanks, 2" of laterite clay mixed with Tri calcium phosphate, gypsum, and sand: all hidden under a 1.5" mix of sand and small laterite nodules.

The CO2 is DIY but a steady one as I have developed my own system which is large (uses 105 L of mix), holds the night production under pressure, and works on a programmed solenoid with a more or less steady pressure.

Recently I replaced 36 W of 8650 K CFL with an unrated 36 W blue CFL, and my algae problem disappeared. Did I effectively reduce my lights without reducing CO2 utility by my plants?

If the blue CFL is an actinic bulb, unlikely as that is, it would reduce the effective light intensity. If it is a bulb made blue by a coating on the bulb, most likely, instead of by use of a different phospher in the bulb, the bulb will emit far less light, and that would also reduce the light intensity. My guess is that you reduced the light intensity just enough to get away from the algae problem.

If the bulbs you use are screw-in spiral CFL bulbs, you can't assume that you have very high light intensity with your 5 watts per gallon of light. If you have very good reflectors for those bulbs you could have high light intensity, comparable to 2.5 watts per gallon of straight tube PC bulbs with good reflectors, and that would be why you had algae problems.

The blue is from the phosphors used and not from any coating. Actually The bulbs used are PL type, twin straight tubes with 4 pins, the ballast is fixed outside the top. I build my own tops and use polished aluminum, coated with clear polyurethane, as my reflector.

I cannot say what those blue bulbs are as they are a Chinese import purchased from the Grey market.

5 watts per gallon of straight tube compact fluorescent tubes is really high light, unless it is suspended above the tank, or the tank is very deep. (My opinion, of course.) The blue bulb probably provided a light spectrum not as useful to the plants, so that reduced the intensity enough to affect the algae growth. But, I'm not sure that algae photosynthesis is the same as for plants, so I could be wrong.