Light level vs. Excel dosing - Tropical Fish Keeping - Aquarium fish care and resources
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post #1 of 6 Old 02-18-2011, 02:09 PM Thread Starter
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Light level vs. Excel dosing

At what light level does supplementation with CO2 become helpful? I am getting mixed opinions. Using T5HO(x2) on a 55g tank [one is a red-spectrum biased "plant" bulb, the other a 6500K bulb], do I need to supplement with Flourish Excel? The lights run 10 hours/day and I am dosing twice a week with Flourish comprehensive. Algae is under control, and the plants are growing quite well. The microswords are putting out runners and the American Val has to have about 3-4 inches pruned/week to avoid blocking the light.

I understand Byron's very clear explanations regarding the balance of factors involved [Leibig's law of the minimum], but is is there a general opinion on when Excel is helpful and when lighting is too low to make Flourish Excel CO2 supplementation helpful? Are two T5HO's too little to need Excel supplementation?

I currently dose with Excel daily. I do know the Excel is not actually CO2, but a metabolic intermediate the plant can use similar to CO2. CO2 is just easier to say!

While injected CO2 is more cost-effective, I have no desire to go there.....yet!


Last edited by DKRST; 02-18-2011 at 02:14 PM.
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post #2 of 6 Old 02-18-2011, 04:21 PM
It sounds like you've got an ideal balance already. I would keep doing what your doing. You have good growth and little algae. But if your hinting at what I think you're hinting at... Then I would also like to know if there is a formula to relate the amount of light, nutrients, and CO2. As far as I know, there isn't one, other than try to find a balance that yields good conditions (which you have).
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post #3 of 6 Old 02-18-2011, 07:18 PM Thread Starter
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It would be nice if it could be reduced to an exact formula, but then it probably wouldn't be as rewarding when it works!
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post #4 of 6 Old 02-18-2011, 07:29 PM
That would be nice!

Hit me up Baby
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post #5 of 6 Old 02-18-2011, 11:53 PM
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Exact formulae are impossible because there are so many factors in the equation. As has been mentioned, you seem to have a balance now. But if it were me, I would want a lower balance, because I am a firm believer in as little light as necessary, and light should always be the limiting factor.

My concern over light is mainly for the fish. Forest fish, which is what most of us maintain in planted aquaria, all (with very few exceptions) occur in rather dimly-lit waters. I posted a video earlier today showing angelfish and cardinal tetra in water that was so dark you couldn't see the fish without a lamp, and it was high noon as the above-water portions clearly showed. Bright light does stress such fish. Baensch refers to cardinal tetra as having a light phobia, and they will not spawn except in near darkness. I expect you get the point.

Like Tom Barr said in a discussion with me about light and algae, one should always start with the lowest light essential for the plants one wants to grow in the tank, and then raise nutrient fertilization--which is not only what we add but what is already there--to balance that light. And back to the formula question, there is none; it is a question of everything balancing, and this depends upon what's in the tap water, the size, number and type of fish, the food fed to the fish, the amount of CO2 from the fish load and the bacteria (more comes from the bacteria in the substrate than the fish), and then the fertilizer added to supply what is missing from the foregoing. Even in my tanks using only Flourish Comprehensive, i have to work out whether once or twice weekly is needed, and i notice it does vary from tank to tank as a result of the fish load, plant species and number, and light.

You mention two T5 HO tubes over a 55g. If these are 48-inch tubes, in my view you have double the light intensity you need. That is very bright light on the fish. I had two T5 HO 6700K tubes over my 115g 5-foot tank for a week last year, and took the fixture back in exchange for a twin tube T8 fixture. The light was way too bright. I could hardly bear to look into the tank, I can't imagine what the fish felt. A good carpet of floating plants helps, but if the light is more than what you need for the plants, light is being wasted which means energy to run the light is wasted, and down the line Excel is being wasted and more nutrient fertilizers are being wasted. Things can be balanced at any level from minimal to very high; which level one chooses will depend upon what one expects or wants from the plants.


Byron Hosking, BMus, MA
Vancouver, BC, Canada

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian. [unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]
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post #6 of 6 Old 02-19-2011, 12:23 PM Thread Starter
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Byron, thanks for the reply to both my threads - I can elevate the current light above the tank some more (it's currently a 24" fixture on only one side, so the fish have a low-light area). I can also use only one bulb, since my fixture has a switch for each bulb. As you said, then I lose the ability to balance the color with two tubes - I can live with that if the plants grow and the fish are in optimal conditions! I'll experiment with the Excel and Flourish comprehensive dosing over time, don't want to waste $$.

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