2 watts as a "rule of thumb" is going to be a very rough starting point. It's a pretty old guideline and is probably based on standard fluorescents. Obviously it will vary a good bit depending on type of light. Nothing is based on incandescent though, as incandescent lights really are not used for plant growth. It will vary quite a bit depending on the depth of the viv, too, and of course with what plants you want to grow.
I have a row 4 vivs being lit with a 4 foot, T12 shop light. It is barely enough for plant growth. I keep it sitting directly on the top of the vivs to take advantage of the limited light. These are actually running on less that a watt per gallon.
I have a row 2-40 gallon vivs under a 4 foot, T8 shop light. T8s are a little more efficient. These guys are still just under a watt per gallon but the better T8's are giving me better plant growth.
I have a row of 4-25 gallon vivs under a 4 foot T5 HO light with top of the line TED reflectors. I'm pretty happy with the light they are getting. We are still barely over one watt per gallon but the efficiency of the T5s is much better and a really good reflector makes a world of difference.
I use very few light loving broms and really high light plants so this has been working for me so far but the T12s really have to go. Future plans include upgrades to either T5's or LEDs as I get into higher demand plants.
But when you get into punching 4 feet down, lighting can be a different story. Standing a 55 on it's end means you only have an 18" surface to put bulbs over. The shorter the fluorescent bulb, the less efficient and bright it becomes. Therefor, my personal guideline is that if you can't fit at least a 2 foot bulb in, T5's may not be the best choice. For this reason, I opted to go with Power Compact Fluorescent for my 75 gallon 3 foot build. (link to viv
Pumilo's 75 corner viv ) (Link to lighting system
DIY rain gutter PC lights) I used 2-65watt compact fluorescent bulbs, putting us at close to 2 watts per gallon. I used some higher light plants for this build and additionally, had to punch 3 feet down. Light on the viv seems to be serviceable for now. Light levels at the bottom are low but most of the plants are wall mounted. Especially at the back of the viv, on the bottom, light level is very lacking. I will most likely need to supplement this with an LED spotlight if I want to see the few plants back there do well.
I just took a 26 watt spiral, compact fluorescent bulb and set it over a 10 gallon vert. To my eye, the light seems adequate. I moved the same bulb (with a reflector of course) over to a 20 gallon vert (2 feet tall) and I find the light very lacking. I think growth would be limited to very low light plants. To take an even smaller bulb than this, and put it over a 4 foot tall viv, I don't think anything would grow or even survive for long, more than half way down the viv. You may be able to grow easy plants like pothos and creeping fig in the top third of the viv.
In summary, "rule of thumb" is an extremely rough guideline that can vary quite a bit depending on type of light, reflector used, plants grown, and the depth of the viv. Hopefully the examples I gave may be a little more helpful.
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Is this enough lighting?
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