I have been thinking about GDA vs. lighting for some time now. Consistently I have noticed that GDA grows where the light on the inside glass surfaces of the tank is the most intense. If no light were to strike the inside surface of the tank my impression is that no GDA would grow on the glass. That is obviously impossible, but it isn't impossible to greatly reduce the amount of light striking the glass.

Tom has noted that pendant MH lighted tanks have less GDA than PC lighted tanks. MH lights tend to have parabolic reflectors, and with the light source being very small, the reflectors tend to be nearly focused, to produce a near parallel beam of light entering the tank. If you have a hex tank, this should almost eliminate the GDA problem?

But, for those of us having the typical rectangular tanks a different approach seems justified. What if we used a single T6 lamp, at the focal point of a parabolic cylinder reflector? And, that reflector would be narrower than the width of the tank, so light spilling over the end would have less chance of hitting the glass. Why wouldn't this virtually eliminate GDA for rectangular tanks?

Of course a problem would be getting sufficient light from a single bulb to grow plants, but two bulbs, each with its own reflector would solve that problem. Here is a bulb that would work for a 4 foot long tank, typically 55 gallons, Maxum 5000 47" F54-T5HO Mini Bi-Pin, and would give about 2 watts per gallon.

Are there any commercial aquarium fixtures that work in this way, or any available reflectors?

But.... this issue is not GDA specific, it's algae and plant specific.
Simply directing lighting elsewhere is a rather obvious notion.

I am not sure why more lighting companies do not do such obvious things for planted and reef tanks.

Having an angled spread vs the tank H and W is a rather simple thing.
Not having direct light hitting the front, sides and rear would be ideal.

But they'd rather rattle on and on about how their "special heavily researched spectrum prevents algae".

They can go suck eggs for all I care.

You can place a screen, light filter, dark plastic etc in front of the glass easily enough and see how light influences the GDA. You can do this on the side so it does not prevent the viewing etc as well. Or all the glass sides etc.

This in itself might be a cure for GDA also. Simply cover the glass, or use plastic/glass etc that will get covered with the GDA instead of the glass behind it, then remove it along with the GDA.

Regards,
Tom Barr

Ah, I see a new market, for fish tanks with tear off sheets like those used on race drivers helmet "goggles".

Well, it's a new market most certainly within the aquarium trade, no light hits the glass, but still hits the gravel/plants, illuminates the fish etc, that is where we and all the other aquarist want the light to go, not hitting the glass.

Very simple idea, but one that seems to have eluded those bozo clowns at the lighting/Engineering depts as well as Marketing depts.

They could sell these for specific tank sizes and make an entire line of reflectors specific for a tank dimension.

But they would rather market a one size fits all light/reflector combo.

Regards,
Tom Barr