View Full Version : How long can I leave lights on a planted tank on?

05-04-2009, 4:34 PM
Hidy Ho good neighbors! I am getting ready to try and grow some diatoms in one of my tanks on purpose. I bought two small sheets of plexi to place in a tank. The tank I plan on using has some plants already in it. From moderate to high light plants.

In order to hopefully coax the diatom growth, I need to leave the lights on for extended periods of time.

My question....how long can I leave the lights on without damaging the plants?

05-04-2009, 4:40 PM
8-10 hours is suggested for plants.

05-04-2009, 5:14 PM
plants will easily tolerate 12+ hours of light per day.

The only reason for most to reduce lights on time is to reduce Algae growth, so if you need 12 hours use 12 hours. what is missing from this all however is light intensity, just how much light are you planning on or working with, and over what area, and to what depth, it all factors in.
But as a general rule if it is not heating your tank water up, 12 - 14 hurs is not out of line. Not needed for the plants, but it wont hurt them either unless your in the 3+ watt range, then you might wish to make some changes/additions.
Like adding CO2, increasing Ferts etc etc etc

05-04-2009, 5:24 PM
There's no reason to have the lighting on for 12 hours a day.

I don't think you can't directly compare photoperiod with light intensity though. 12 hours with 30 watts is not the same as 6 hours with 60.

05-04-2009, 5:59 PM
7-10 hours for plants, I have a photo period of 8 hours in my tanks.

05-04-2009, 6:40 PM
They can do 8-12 hours. Although I like to do only 8 hours a day, sometimes 10. It really depends on my mood. Going over 10 seems a little excessive to me and it generally starts algae problems if it's a medium-high light tank.

Low light tanks are able to tolerate longer photoperiods but even then, eh... it's iffy. I don't see the point in keeping it on so long anyway. Waste of electricity :-P

05-04-2009, 7:06 PM
My normal photo period is 10 hrs. I have DIY CO2 on all my tanks and dose ferts as well. The need is to actually (believe it or not) encourage diatoms to grow on a piece of lexan that I placed in the tank.

05-04-2009, 7:16 PM
What kind of diatoms? Green spot algae? If that is the case, higher lighting will help a lot. How long the lights are on doesn't seem to affect growth so much and would end up encouraging other algae to grow instead. Green spot is pretty normal to have, to a certain extent.

05-04-2009, 7:20 PM
brown...typically a good source for otos.

05-04-2009, 7:24 PM
Oh, brown? Ugh, that stuff is annoying.

Easy way to trigger a brown diatom bloom would be to just stir your tank up a lot and cause a mess. If this is an established tank, which I would assume it is already, then increasing light intensity or hours won't do much. Causing a mini-cycle can trigger brown diatoms to appear too.

Not sure why you'd want to do that though. It'll cause more problems than it does help your otto's diet. Algae wafers and other natural algae in the tank should suffice in terms of keeping them happy and healthy.

05-04-2009, 7:24 PM
well the OP originally asked if it was afe to leave lights on for the duration he needs to grow diatoms

the answer is yes as the aquatic plants most of us grow are easily able to deal with recommended photo periods of 10 - 12 hours...
a great many even split this 10 - 12 hour time into 2 cycles to further disrupt the Algae growth cycle


Excerpt from the above article:

The duration of time the aquarium is lit is often referred to as the photo period. For most aquatic plants the photo period should be 10-12 hr. per day. Using a light more than 12 hr. will not compensate for weak lighting nor will it produce better growth. In a low-tech aquarium darkness is nearly as critical as the photo period. This is the time plants can build up the CO2 supply in the water they will need the following day. There also seems to be more growth at night.

The original queston being how long can he leave the lights on without harming the plants.

as he neither included types of plants, depth of tank weather he is injecting CO2 which could all play into it, a simplistic answer is 12+ hours .

12+ hours maybe not be needed for optimum growth
but neither is it going to hurt them. Now if he wished to go longer you have to find a way to help the plants achieve their photoperiod results ie: Adding CO2 as plants create CO2 while consuming O2 at night and if you are reducing the "Dark period" you have to help the plants along by providing them with CO2. If you are providing them with CO2 and an abundance of light, you will also be stimulating their growth which also means you need to Feed them more.

The intensity would only come into it if he plans on running the lights bright enough to potentially "burn" the plants near the surface.
none of that info was included. only

The question was simply:

Can i run the lights for extended periods with out harming the plants.

answer... is not so simple but 10 - 12 hours should be no issue as at that length of time is a normal interval for aquatic plants. any thing beyond 24 is doable, not needed but doable, and with out harm, .....

doing that forever .... well now there is a good question.

05-04-2009, 7:26 PM
There's no reason to have the lighting on for 12 hours a day.

I don't think you can't directly compare photoperiod with light intensity though. 12 hours with 30 watts is not the same as 6 hours with 60.

there is usually no reason.. because most of us want to grow plants not algae

but... The OP is TRYING to grow things that most of us grow by accident... so the more light the better

05-04-2009, 7:28 PM
If the main question is, "Will long hours harm plants?" No, it won't harm them directly. It will, however, cause some types of algae blooms (not diatoms) that will harm plants in the long run. So in a sense, too many hours of light per day can harm plants through algae, although not directly...

Light -> Plants = Plants are just fine
Light -> Algae -> Plants = Light grows algae, algae (can) harms plants.

Does that make sense? :)

Brown diatoms won't appear though. That's probably on the bottom of the list of algae that will appear first if you keep lights on for a long time. You'll probably see staghorn and BBA first. Green spot and hair after.

05-04-2009, 7:30 PM
Yep, more algae means less nutrients available which means less growth potential for plants. So it depends on which you want more.

05-04-2009, 11:48 PM
maybe you could try putting in a little small bit extra ferts then usual, that might also trigger a algae bloom

or is that not something that should be done? (not talking alot extra, just a small amount)

05-05-2009, 12:13 AM
if you really want diatoms start a new tank dont try it in your established tank

05-05-2009, 4:28 AM
if you really want diatoms start a new tank dont try it in your established tank

:iagree: And you need to go low tech. Not high lighting and no CO2. With low light, low tech, I doubt you're going to see other algae blooms. And I can only base this on my low tech tanks with low light plants. My 75G planted pleco tank and 55G sids tank are both low light, low tech and because I'm still growing out the fish in there and need to feed 2X a day, and have to leave the house very early for work, I've been turning those lights on at 4:15 AM every day and they don't get turned off until somewhere between 10-11:00 PM. They are established tanks so I don't have diatom growth, nor do I have green spot or much of anything else. I do have a few very tall plants like crypt usteriana that has huge, long leaves that fan out over the surface of the tank and those leaves grow some algae....staghorn I believe it is, but I just pluck the little clumps off when I do a water change. Since they are at the very surface of the tank and only inches away from the lights, there's nothing I can really do to stop it except weekly dosing of excel which I'd rather not do. But it is not unmanageable at all. With low tech I'm only dosing Flourish once a week, most weeks after a complete water change and if you've ever seen my crypt collection, I think you'd agree long periods of low light in a low tech plant will not harm plants or grow a ton of nuisance algae. Now, I do have a double strip 48" light on the 75G tank....if I turn both bulbs on.....I'm in trouble with BBA and staghorn.

05-05-2009, 3:25 PM
if you really want diatoms start a new tank dont try it in your established tank