So I'm now down to 8 hours of CO2 per day with no algae. Is there a rough correlation between lighting duration, intensity, and CO2?

The tank is 75g, heavily planted with 6*54w HO T5 daylight bulbs. I decided to slowly cut down on the hours of full beam on the basis that the sun is not directly overhead for most of the day. I done similar with my reef.

For the last three months I have been running two bulbs for 11 hours and the full six bulbs for only 5 hours. Since the tank is stable I reckon I could cut down to four hours of full light and maybe 5 hours of CO2. Am I chancing it?

Why cut down the hours of CO2? As long as there is light the plants will use CO2, and the lack of CO2 will be a problem. Granted that the amount of CO2 in the water doesn't drop to zero just because you shut off the supply, but I don't see what advantage there is to shutting it down more than an hour before all of the lights go out. And, there is certainly an advantage to having it on before the lights come on at all.

After three months I would have expected to see a problem by now. The tank is low light for most of the time and I'm only injecting CO2 during the high light period (and slightly before and after).

I've run the tank for years with the standard CO2 for the whole daylight period, and now I'm trying something different. What have I to gain? Nothing really except less refills and less trimming time.

If I get an algae outbreak I'll rethink my strategy. I'll make a post if that happens, but until then I'm happy with the way it is.

Why would you really need high CO2 when you are only using 1.4 w/gal?
Will adding CO2 help?
Sure.

Will it make the system more or less stable?
Most likely more stable, thus............why destabilize it?

That's a very obvious question.

If it's for test purposes, think about the test first.
Think about what your question is.

That's a different question than whether it's more stable without CO2.

Why add so much light in the middle of the day?
4x54w T5's can grow any plants and that will reduce the trimming and CO2 demand and growth rates.

Why use all 6 if that is really your goal?
You can measure the light in aquatic systems easily with a light meter and no, after after 1-2 hours, the light is far more intense than folks are assuming.

800-1200micromols for about 10 hours out of a typical 12 hour cycle.
Far above the levels we have and that aquatic plants demand(which is very low).

You might also try running one back of the T5's for 3 hours each, front to back etc.
That would give you good spread and lower light intensity.

Thus less growth. less trimming and less CO2 demand and a more stable tank.
While also providing with thicker growth and foliage.

Also, how is CO2 for 5 vs 8 hours better/worse?
CO2 is cheap, we add it to increase growth rates namely.
You might save 7$ per 20lb tank based on a 20$ refill cost.
A 20lb tank ought to last 2-4 years at least on a 75 gallon tank, you save: 3.50 to 1.75 per year.

I guess.

Tropica showed that low light tanks gain benefits from adding CO2 as well.
Lower light makes the CO2 demand go way down, so supplying enough CO2 is a lot easier.

If you reduce light down far enough, then you no longer "need" to add CO2 at all, is that somehow of interest to you?

If so, try some non CO2 tanks.
They grow plants and reduce the trimming efforts etc.

Ask yourself why you add CO2 to begin with, most plants do pretty well without adding it, they just grow a lot slower...........



Regards,
Tom Barr

More out of necessity. Nine tanks in a small room makes for heat issues so I allow myself one high light tank (excluding the SW) with injection. Now, I know some people can grow whatever they want under low light. I've tried but can't get the same effect. I know my Bacopa australis doesn't creep along the substrate in my low light tanks.

lol I guess what I'm doing isn't too popular. Have I finally lost it

Popularity has nothing to do with it.

Heat does...........
So with that in mind, using less light does really seem like a better solution.
There are many species one can use, why change everything and force yourself into a situation for one plant?

There are going to be trade offs for every situation.
Addressing those trade offs and goals is another matter.

The trade offs might be worthwhile for you if you have a certain goal, however, often such trade offs are more like a barrier that folks put up in their own way due to frustrations or belief.

I must have high light!

Try 2-4 bulbs instead of 6, I'm certain there's not a plant around you cannot grow just fine using 4x 54T5 bulbs on a 75 gallon tank.

Having more of a potentially limiting nutrient seems like a better approach than less for CO2.

At least as far as stability.

Regards,

Tom Barr

Well, not just the bacopa. Ammania gracilis and L. glandulosa come to mind. I have to admit though that I do like the brightness of the tank with the six bulbs on. Next to the planted (high light) SW it looks pretty sharp.

Steve grew awesome A gracillus as did I and Erik with about 2-3 w/gal.
Very pretty, same with L granulosus.

Many like to claim HC is a high light plant which it's not either.
You'll note, many folks outside the USA use much less lighting and somehow manage.
They also grow these same species somehow without the higher light.

Now if you just like brightly lit tanks, that's fine, as well as adding CO2, but you cannot reconcile that with a desire to slow growth at the same time........there is a trade off there.

Still, if light is really the reason, how is it that many folks can grow these plants with less light and have good results?

I've grown these plants at much less light than you have and know they can be grown well at less light.

2 w/gal was high light back 15 years ago

Regards,
Tom Barr

I grow A. gracilis in a tank with 1.5wpg. No problems at all. I threw in a few spare cuttings and was surprised that they grew. They don't look quite as good as the ones in the high light tank. Thinner stem, smaller leaves, but the coloring is still nice.

I had pretty red color at low light actually, which is not what many would assume.

The tank had few fish and relatively low NO3, I add more and did not get much color change response.

At low light, the plants does not stunt nearly as much and is able to survive pretty well.

Folks that like lean nutrients are wise to use lower light to balance their lower nutrients...........

Higher light to balance with higher nutrients/CO2.

As long as you are adding a good match at the right time, there's no reason adding CO2 only at the high light peroid will not work.
It should in theory and apparently in practice for you.

But...........what would make the method you use even more stable and robust?
What have you learned from high lighting?

Good stable CO2, good stable nutrients, routines like water changes and cleaning/trimming, good current etc.............

When you apply those same methods to a low light tank, now they are not as critical, but they still offer and wider range of stability: much more wiggle room with respect to dosing, targeting a range, less algae etc

Which most every one says they desire..................I do not know anyone that does not desire that goal.

There's a lower end to that to be sure, but it's a lot lower than many give it credit for.

Regards,
Tom Barr