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Lighting Deep Tanks

4341 Views 5 Replies 5 Participants Last post by  MiamiAG
I heard if you have deep tanks(24") it's better to use MH than PC, is this really true? If PC were used would this cause Glosso and the such to go bad? Would having a deep(4") substrate solve the problem? :?
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A 24" deep tank at Albany had no issues using 2.1 w/gal of lighting, some PC some NO FL's growing all sorts of plants including Gloss, Rotala macrandra, Eustralis etc.

Tom Barr
I don't know how much I believe that a couple inches makes such a big difference. Unless your water is tea colored, a couple inches are not going to make much of a difference IMO. Specially considering that the light inside the tank mostly reflects off the glass walls rather than penetrates it and is lost. I think the major issue with taller tanks is other plants creating shadows. If I'm right, then a point-form light such as MH would actually increase the amount of shadows compared to a light source that is more evenly spread out accross the surface area of the tank. Just like sports fileds have multiple lights to avoid shadows, something similar can be said about our tanks. When I moved my 96W fixture (3x32W) from the 55 gallon to the 90 gallon, it really didn't make a lot of difference to the lower plants, they seem to grow just as well in the 90 as they did on the 55. Even when I changed from a 10 to a 15 high I can't say I noticed much difference in plant growth, but the plants are now able to grow taller in both tanks and therefore shadowing is a bigger concern for the lower plants.

I may be wrong, I can only speak for the 4" difference I have experienced myself and it didn't seem like much changed at all. I'm about to finish a MH fixture but I doubt I can compare it to what I have now, 100W of MH emits more light than 100W CF so the comparison is probably not very fair.

Perhaps reflector efficiency could make a bigger difference. I'm sure a good parabolic reflector on a MH bulb can penetrate far deeper than any linear reflector, MH or CF.

Giancarlo Podio
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Maybe with regular old fashioned T12 fluorescents, but with the AH Supply reflectors I think CF lighting is pretty intense.

The 175 Watt MH pendant lights are focused more, so you will have one area slightly brighter. However I think the dropoff side-to-side is greater with MH. People used to say they liked pendant MH because it went better with open top tanks and the ripples in the water would create shimmering shadows on the substrate, and feel this is more natural. But because of this focused aspect there are shadows in the tank where ever there is a leaf. The light doesn't diffuse to the bottom under the leaves like fluorescent light. Therefore with MH pendants you have to be very careful about floating and emergent growth blocking light below. Even one stalk of say Bacopa carolina can block light to put the bottom of the plant in shadow.

With the 55W and 96 W CF bulbs coupled with AH Supply's super reflectors, I think that you can put together a combination that will punch through all that water and evenly illuminate the tank.

I have a 125 gallon tank that is about 22 inches high. I have a soil substrate which is 4 inches thk by necessity. It does help. But then did you buy the tank because of its depth? In this case just worry about getting your hands down to the bottom of the tank to plant those critters. :wink:

Steve Pituch
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Does the added depth of the substrate a factor or is it the volume of the substrate that is better for the plants and has nothing to do with the lighting?

I think what has been said about shading does play a significant role when comparing PC/linear tube FL bulb's vs MH's.
I've had both for many years.

My solution is to use the HQI double ended small 150w bulbs closer together, these take up less room, look great and provide good spread vs the ugly MH Globes.

A better MH reflector certainly helps but you still have self shading issues that are not as significant as with long PC tubes etc.

I would even say that the PC bulbs cut off at the ends and the middle of 2x55w on a 4 ft tank are significant vs a T-5 bulb but these are still too expensive in the USA along with HQI double ended lighting.

But the cost will come down later.
150$ for a 150w small HQI double ended pendent that looks much better than those Hamilton/Corallife globes(which go for about 250$ ea).

Electronic ballast will extend bulb life considerably and also increase output and reduce noise down to zero.

But these cost more,
Tom Barr
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I'm posting a response George Booth gave on the APD back in 1995. I think he has a firm grasp of the factors you need to grapple with in lighting a deep tank.

Don't think so much in terms of inches but in terms of lumen/lux/par at the leaf.

The discussion on light requirements is tending toward opinions based on "What works for <insert your name here>". This is all well and
good, but doesn't provide much basis for determining "what will work
for me" based on "what works for you". There are quite a few variables:

1) Your tank size
2) Your water conditions
3) Your maintenance regimen
4) Your specific plants
5) Your aquascaping particulars
6) Your criteria for "success"

The simplified "watts per gallon" criteria may be useful for beginners
to show them that the 15 watt bulb that comes with their 29 gallon
tank is insufficient, but doesn't really help that much. A more
pseudo-scientific method is sorely needed that brings together the
known parameters. Let me try to start a discussion by rambling a bit.
Feel free to comment.

Success criteria

A "successful" plant tank can mean almost anything. Newbies may be
satisfied if the plants don't die right away. Casual aquarists might
be happy of the plants stay green and they don't have to weed the
tanks every month and don't need to mortgage the house to buy
fertilizers. Enthusiasts want actual growth and propagation so they
can impress their friends and win HAP points. Fanatics like me want
massive growth and aren't happy unless the plants are supersaturating
the tank with O2 and can be harvested for sale at plant auctions to
pay for the high tech products being used.

The oft-maligned Kevin Osborne is perfectly happy with his smattering
of non-demanding plants. I would be bored to tears.

I wonder if we could develop some objective terms to describe our
successes? Somehow I think the results obtained with a gro-lux and
cool white bulb and a bag of breath are different than results
obtained with obscenely expensive triton and ultra trilux bulbs and
automated CO2 injection. Both techniques are satisfying to their
proponents but need to be quantified somehow for someone trying to
decide which route to go.

Light requirements for plants

Different plants require different amounts of light energy for growth.
Growth is limited by the amount of carbohydrates that are created
during photosynthesis. Photosynthesis is limited by the amount of
light energy available (required intensity and spectrum) and the
amount of nutrients and CO2 available. At a certain threshold of
energy, no photosynthesis will occur. Above that threshold, more and
more photosynthesis occurs until an upper threshold is reached. See
the discussion in "Dynamic Aquaria" for more details.

There is precious little information about how much energy various
plants need. The only quantitative data I've found is in "The Complete
Book of Aquarium Plants" (Allgayer and Teton, translated from French).
The following table is from that book (roughly treanslated):

Amount of lighting in lumens/m^2 (lux)
Plant genera 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
surface plants |------------------------------------------->
Myriophyllum |--------------------------X----|
Bacopa |--------X---------|
Hottonia |-------X--------------------------------|
Ludwigia |-------X---------------------------|
Aponogeton |--------X----------------------|
Ceratopteris |---X-----------------------------|
Nomaphila |--------X---------------------|
Rotala |--------X------------------|
Vallisneria |-------X--------------|
Echinodorus |--X-------------|
Sagittaria |-X---------|
Cryptocoryne |-X--------|
Marsilea |-X--------|
Micorsorium |-X--------|

"X" indicates the optimum for each genera.

Depending on the plants in the aquarium, you can decide how much light
intensity you need *at the plant*. For a tank holding Ludwigia,
Rotala and Nomaphila, the plants would need over 1000 lux at the
leaves. Foreground plants like E. tenellus would need around 600 at
the leaves. The upper leaves will get plenty of light so you need to
consider the needs of the lower leaves. Once you know the intensity
required at the leaf, you can figure out how much you need at the

Light loses intensity very quickly in water. What we need is a chart
that shows how much is lost so we can determine how much light is
needed at the surface to produce the required amount at the depth we
desire. The Allgayer book has such a chart but it was apparently
garbled in the translation and is useless. There are aquascaping
specific considerations, also. Do tall plants overshadow forground
plants? Are emersed leaves blocking the light? Also, light is
reflected from the aquarium sides so that plants near the glass get
more light than plants in the middle. Perhaps such a chart could take
"typical conditions" into account. Perhaps actual measurements with a
luxmeter could be taken.

If you know how much light is needed at the surface, you can determine
approximately how many lumens you need in the hood. If all the lumens
from the bulbs could be focused onto the water surface, the lux at the
surface is (total lumens / area of surface in meters^2). Derating
factors such as reflector efficiency and hood configuration could be

You would also need to take in account the reduction in lumens over
time as the bulbs age. If you designed your light system for just the
right amount of lumens based on the initial lumen rating, it would be
suboptimal a short time after startup and would never be right unless
all the bulbs were changed at once. So a factor considering something
like 25% new bulbs, 25% almost new bulbs, 25% getting old bulbs and
25% old bulbs would need to generated.

Hmmph, maybe "2 watts per gallon" isn't so bad after all ;-).

As for light type, there is no clear data that indicates whether
blue-red or full spectrum light is "better" for growth. I prefer the
appearance of full spectrum light. The intensity of bulbs is
expressed in "lumens" which is weighted towards humnan sensitivities.
There are no commerical ratings geared towards plants so we are stuck
with lumens. The output in lumens of a gro-lux bulb looks poorer than
a full-sepctrum bulb even though the "useful" light energy *may* be
the same (I don't know for sure). But anything we come up with will
have to deal with lumens with perhaps some factor for the bulb type.

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