Old Bulbs = Algae
 

I understand that older bulb lose intensity and there's a slight spectrum change. I've heard that old MH bulbs can be a contributing factor to unwanted algae growth. Could someone explain the theory behind algae growth?

The reason for the algae growth is because most bulbs will actually have a fairly significant spectrum change. The spectrum goes red, yellow, white, blue. Not sure what red is (0K?), but yellow is 5000K, white is 10,000K, and blue is 20,000K. Everything in between is in between (go figure!). Whatever color it starts as (blue or white) it will fade towards the red spectrum. A new bulb being 20,000K may be 16,000K after a year. A new bulb being 10,000K may fade to 7,000K. Each bulb will fade a bit more or a bit less than the next bulb, and the ballast will also influence. The red spectrum encourages plant growth, that is why horticulture bulbs are low Kelvin like 5500-6500K.

What effect does age have on T5 fluorescents?

Same thing, but quicker. It's the same for all bulbs, including household bulbs. To avoid this spectrum fade it is wise to change bulbs at certain times according to the type of bulb:

Halides 12 months
T5HO 8 months
Compact Fluorescent 6 months
VHO 12 months

Over-driving the bulbs will cause them to fade faster, like say using a mogul halide bulb on an HQI ballast. Using fans on the bulbs to keep them as cool as possible will help them perform better in their usable life as well.

The spectrum and intensity does change in various lighting bulbs but its not the major causes of an algae bloom. Algae will grow because there is an access of nutrients. You could put a ton of red over a tank will low nutrients and no algae will grow. In fact that’s what people are doing with the new fiji purple and ati purple plus bulbs, they are adding red spectrum. If you have excess algae growth you will have access nutrients (nitrates, phosphates, silica, etc). However, the added red and yellow spectrum will make a high nutrient tank grow algae much faster because algae use red spectrum very well for photosynthesis.

Now my algae growth makes sense, lol

Not exaclty the answer to what the OP asked but I thought I would post this for curiosity's sake. The colour temperature (as some may know) corresponds to the peak wavelength of the radiation of a blackbody at that temperature. To find this wavelength (should you be so inclined), you can use Wien's Displacement Law:

wavelength (meters) = 0.002898 / temperature (Kelvin)

So a peak of 700 nm (very red) corresponds to about 4,140 K.

To make it really simple. Algae need a few things to survive and thrive.
-They need light to photosynthesize (especially red light or low kelvin color light like 6,500k or lower)
-They need nitrates, fish crap and decomposing waste, some water sources can have high nitrates to begin with
-They need phosphates, most flake and frozen food, sometimes from tap water or a bad ro membrane.
- And sometimes certain algae use silica to their advantage (mostly diatoms)

If you are getting an algae bloom and your bulbs are old it will not necessarily fix the problem. It might help if you get new higher Kelvin bulbs like 14,000k or 20,000k but they real key to eliminating algae is to lower your nutrients. Try reading about carbon dosing, increase your water changes, feed less, get a bigger skimmer, etc.

Thank you for posting this, but it doesn't make sense...? I have some household bulbs that say 3500K on them and they are not red. Am I missing something?

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Chris, the OP didn't ask "why do I have algae?". The OP may not have any algae at all. The OP asked "I've heard that old MH bulbs can be a contributing factor to unwanted algae growth. Could someone explain the theory behind algae growth?" There are nuisance algae threads all over the place (my signature for example lol).

Doesn't make sense either. Graphs I've looked at show red being at 1000-2000k