Why small tanks need more watts per gallon

[talontsiawd]

Very interesting. I would have thought that height would be a "limiting" factor but it makes sence that a few inches may not be nearly as important as how "wide" the light is (which is evident on my 10 gallon with spiral CF bulbs). How many lumens a plant needs being non linear to tank size makes sense as well. I'm not even sure this is what was meant, i'm still totally confused about the whole thing but good thread.

[Shad0w]

All are so confusing . what I normally do was just put as much as light set that can fit on top of the tank. My 3ftx1.5ftx1.5ft tank have 6x39W T5HO on top. The down side is I pay more electricity bill, dose more fertilizer, inject more CO2.

It may not be the best way but save my headache

[Hoppy]

Time has been marching along since I started this thread over a year ago. Since then, like most of us, I have learned a few things. That makes me believe that the conclusions in this thread are not accurate at all.

The mistake I made, and which most are still making, is to accept that the wattage of a bulb has something to do with how much light that bulb produces in lighting an aquarium. It doesn't. Any type of bulb, from T12 to T5HO, produces about the same light intensity directly below it, whatever the wattage of the bulb is. In other words all T12 bulbs produce about the same intensity directly below them - the higher wattage bulbs are just longer, so they produce that intensity over a longer aquarium. Place a 40 watt T12 bulb or a 20 watt T12 bulb over a ten gallon tank and you get the same intensity of lighting.

Small tanks don't require more light than large tanks, and very large tanks don't require less light than medium tanks. The higher the tank the more light it needs to get adequate intensity at the substrate. The greater the front to back depth the more bulbs are needed at the top of the tank to get good uniformity of lighting throughout the tank. It is that simple.

[Church]

Originally Posted by Hoppy

Any type of bulb, from T12 to T5HO, produces about the same light intensity directly below it, whatever the wattage of the bulb is ... It is that simple.

You can't be serious, right?

I've never even seen this thread before now, but I'm about to read it...

[nismo tetra]

I am going through this same "smaller needs more" frustration with my 5.5 gallon. I have 2x9w 6,700k CF over it. Problem is the fixture is not long enough(6 inch bulbs) so the edges of the tank are significantly darker. Now last night in my junk pile I found a 16 inch single tube fixture that uses a 15 inch T8 14w bulb. It lites the tank much much better, but I'm worried if it is intense enough to grow my plants. I need to get a new bulb, which they are somewhat hard to find. But I did find a GE 9,325k 14w bulb that I think I may try out. http://www.drsfostersmith.com/produc...10&pcatid=3810

Another question on that, it has a ballast in line with plug. It's an oceanic fixture (could be 15+ years old lol) and on the inside of the light fixture it says 22w. Does that mean I can use a 22w bulb if I can find one?

All I have in there is some elocharis vivipara, elocharis parvula, and some mayaca fluviatilis. Was wanting to maybe try some HC on the top of the tall rocks (few inches under the light fixture). Not sure if it will get the job done though.

Do you guys think that single bulb would be sufficient for the plants?

[Hoppy]

Originally Posted by Church

You can't be serious, right?

I've never even seen this thread before now, but I'm about to read it...

Yes, I'm serious. Notice that the wattage of a fluorescent bulb is approximately proportional to its length. A 48 inch T5HO bulb uses 54 watts, a 24 inch bulb uses 24 watts, a 60 inch bulb uses 80 watts, or 1.1 watts per inch, 1 watt per inch and 1.1 watts per inch. As the bulbs get shorter than that, the non-tube portion of the bulb becomes a much larger percentage of the total length, so the relationship breaks down. In T12, a 48 inch is 40 watts, and a 24 inch is 20 watts. The bulb length just determines how much power it takes to drive the bulb, not the brightness of it.

[lauraleellbp]

Now I understand what you're saying, Hoppy. At first I thought you were saying the light intensity would be the same under a CF vs T5 vs T5HO bulb as long as they were all the same length, and that wasn't making sense to me.

[Church]

Well then it would seem that running 24" fixtures over a 10g might be the best option to have choices with, in terms of whether to go low-light or high light, or anywhere inbetween. I mean, who cares if the fixture hangs over the edges of the tank by 2" on each side, right?

Hoppy, you said yourself that the relationship breaks down as the bulbs get smaller, so maybe that is directly related to the need for larger wattage on smaller tanks? That's why I'm saying maybe using longer bulbs might open up better possibilities.

Just thinking out loud...