UV sterilizer/filter and a couple of other questions.

UV sterilizers are unneccessary in my opinion. They can be helpful, but only in very limited situations - green water, bacterial blooms, etc. I think a better place to spend that money is on lighting, filtration, substrate, plants, fish, and the tank itself.

You can make a very crude guess at the right height for the pendant by assuming: the intensity will drop inversely proportional to the square of the distance to the bulb, and about 2 watts per gallon when located around 2 inches above the tank would be good if the tank were only 18 inches or so deep. You will have 4.2 watts per gallon, located X distance above the tank. The distance to the substrate will be X + 24 - 4(substrate depth), or X + 20. The "standard" tank would have the light 18 + 2 - 4, or 16 inches above the substrate. [16 divided by (X + 20)] squared times 4.2 = 2. Solve for X: about 3 inches.

That gives you a feel for how high the pendant should be. Obviously the answer is not two feet, or even one foot, but closer to 2 to 6 inches.

This will give a very high light intensity in the upper portions of the tank, but just high light intensity at the substrate level. Just keep in mind that this is a very crude method for judging the light intensity.

 

Hoppy,

Took me a while to understand your formula. I wasn't grasping that the 16 ("standard" tank distance to substrate) was a constant. I did the formula up in another way:

d = sqrt( (256*lwpg)/dwpg )

d = distance from light to substrate
lwpg = lighting wpg
dwpg = desired wpg

If you fill in lwpg and dwpg and solve for d, you have the distance from the light to substrate. I think I'll put this in a spreadsheet program for future use.

Thanks.

 

In case anybody is interested in calculating what their wpg is at their current distance of their lights from the substrate, here is the formula:

cwpg = 256 * lwpg / d2 where

cwpg = current wpg
d = distance from light to substrate
lwpg = lighting wpg

Hoppy (Vaughn) did say this was just a crude method, but it's better than just guessing.

I did up a spreadsheet with both of these formulas (MS Excel), in case anyone is interested.

 

Remember, this was for a 24" cube, and the 16" standard depth was a crude way to include the non-standard geometry of that cube tank. If you were dealing with a 40 gallon tank, for example a bow front tank, this wouldn't be the right relationship. Also, the drop in intensity with the square of the distance is only good for other than tubular lights which span most of the length of the tank. For those lights the drop in intensity is more nearly proportional to just the distance, not the distance squared. When I called it a crude way to estimate, I really did mean crude.

 

I'm glad that formula isn't perfect for tube lights because I was looking pretty bad on a couple of my tanks. I don't suppose you have a "crude" formula for tube lights, do you? I tried to find one on the net before, with no luck. I don't know how much those PAR meters cost, but I doubt that I can afford one! That's really the way to go...

 

"Crude" Tube Lights Formula:
Under a tube light or a pair of tube lights the intensity drops off at something between an inverse square and an inverse direct relationship to the distance. In the middle of the tank, you could estimate reasonably well assuming that the intensity drops off inversely proportional to the distance. Tube lights are most likely to be used on tanks of close to standard geometry, not cube tanks.

So, assume you have a couple of T8 bulbs, with good reflectors, over your tank, giving "I" watts per gallon. A high light intensity would be about 2.5 watts per gallon or above. Assume the tank depth is "D" and that you have 4 inches of substrate, and the bulbs are 2 inches up off the top of the tank. That puts the bulbs D+2-4 or D-2 above the substrate.

Raise the light above the tank "X" distance. Now the light is D-2+X above the substrate, so the intensity at the substrate is (D-2)/(D-2-X) times I.

Let "J" equal the new intensity. J = I * (D-2)/(D-2+X)

Solve for X = (D-2)[(I/J)-1]

If original intensity is 3 watts per gallon, and you want to reduce that to the equivalent of 2.5 watts per gallon, X=(D-2)[3/2.5)-1], or X = 0.2(D-2)
If tank is 20 inches deep, X becomes 3.6 inches.

 

Thanks for the formula. I was just thinking that a friend of mine probably has a PAR meter. Maybe I can borrow it to help set up my new low light tank. Tom once mentioned a low light tank being like 100 micromoles near the top and 30-40 micromoles down lower. Setting that up would be quick and easy with a PAR meter.

 

If you try this you will find that it is anything but quick and easy. First, the intensity will vary greatly as you move the meter sensor from the center of the tank to the ends or to the front and back. It will vary greatly depending on the depth of the substrate - if it is sloped front to back, that will affect the reading. And, are you going to judge the light intensity based on substrate level readings or an inch above the substrate or two inches, etc.

It is very interesting to observe PAR readings in the tank, and you can learn a lot by doing so, but in the end you will still be doing some crude estimating in rating your tank as low light, moderate light or high light. Perhaps we should think in terms of the log of light intensity - much more "friendly" measurement.

 

Wish I had read that BEFORE buying, installing, and removing one (due to leaks) Oh well, live and learn...

BTW, I have 3x150w MH over my 180 and they are currently set 13" above the surface. While some may think this is an unlucky #, my growth has been excellent at this height. However, now that I see the rate, I may raise a bit to slow it down. A great problem to have

Lowering the light increases growth rates in my setup also, as Tom states.