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Light intensity vs light hours?

Ariel S

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ive been struggling recently with various algae blooms in all of my tanks. I’ve been trying to figure out a balance and I’m hoping I can get some help.I’ve had these tanks running for years but the last few have just been algae farms and my plants are certainly suffering. 

im trying to figure out if I have too much light or too long lighting hours or not enough light/too short of hours.

right now I’m using Fluval 4.0 running at about 30% ish for about 6-7 hours a day. 
I’ve  tried many different combinations but just can’t get it right. Constant algae farm.

Photos attached. 








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On 11/28/2022 at 3:49 PM, Ariel S said:

im trying to figure out if I have too much light or too long lighting hours or not enough light/too short of hours.

There's always "a few things" going on when a tank gets this way.  It's unfortunate and it's always difficult to figure out perfectly, but let's break it down.

To your question. and there is definitely other people that can explain this a lot better than me, algae requires a certain amount of time (duration) to grow and reach a certain size.  It can do so at what we would think of as relatively low light.  Some algae can thrive under very intense light conditions (like greenwater).  Funny enough, if you just put a light over a tank, you might not get any greenwater.  It happens and it's not so straightforward. 

That being said, plant like a duration, longer exposure of light, but can grow under relatively short exposures of intense light.  Think rainforest and how the plants on the canopy block out the other plants, but they still can grow and adapt to that environment.  So.... sometimes when you want to fight algae your best bet is to reduce exposure time (duration, 4 hours or less) and intensity (your intensity is fine, I have one tank running at 15% right now).

Next we have to consider the plants themselves.  Something like anubias, java fern, might need nothing more than to be in a room with a window and that alone is enough light.  Some situations, that's perfectly fine.  That being said other plants like pogostemon stellatus octapus or jungle val really do well with stronger light.  Again, not about duration, but that intensity you might need for those plants to do well.  Some carpeting plants will want very strong, intense light to reach the substrate and allow them to grow outward instead of trying to grow tall towards the surface of the water (towards the light).

There's little tricks to lighting, but hopefully that helps things out.  I learned a lot from this talk as well as many other channels on youtube.  I'll toss the link below and hopefully that can help you out to understand your light settings.

For my own 29G planted tank, these are my two profiles I've used.

The 36" I have had as high as 55% without much issue, but much like your own situation I am dealing with a very severe (and difficult) algae right now.  I have my window down to 4 hours, staying with the current intensity, trying to get rid of the BBA/Staghorn I'm dealing with.


Prior to upgrading to the larger light, this was my profile on the 24" light.  Once I went higher than this intensity the algae took off.  I had it turned up to try to reach the S. Repens on the substrate.  I ran the light at this intensity for ~3 months and was at or near this intensity (flipping between 65%, 75% and 85% for about a year).  I would not go higher than 75% for this specific light.  If you have higher demand plants, then get the 36" version and start at about 30-40% (33% more powerful)


Edited by nabokovfan87
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There looks like a lot of mulm build up in a few of those shots. That's an ammonia source which is probably the biggest contributing factor to the algae problem. I would try to increase water turnover (filtration) or increase your maintenance cycles and water changes. 

After that's taken care of, keep up maintenance, water changes and fertilization. See what happens after about 3 to 4 weeks 

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Light actually has less to do with algea than most people think. Let me scream it for the people in the back, HEALTHY PLANTS NEED A HEALTHY AMOUNT OF LIGHT, in turn lots of healthy plants are your best defense against algea. I would first start by cleaning up the dead organics/mulm in your tank. Concentrate on water quality as mentioned above,  (water changes are your friend)! Add a clean-up crew to help, nerite snails, ottos, ect, think growing an eco system not setting up a fish tank.... Then plant heavy, more healthy soldiers  equal an easier fight against algea. From there you adjust your light based on plant reaction to light settings. Don't believe me? I run my lights for an insanely long period of time 13+ hours and I have a relatively algea free tank thanks to abandoning the conventional fish tank wisdom of light equals algea and adopting the "build your eco system" mantra when dealing with a planted tank. Pics of my current tank are in my journal linked below


Edited by JoeQ
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its all a balance. light and nutrients. with lighting you are correct with how long vs intensity. lower intensity you can run much much longer, high intensity a shorter time. if the nutrients are there, too much light will grow algae. if the nutrients arent there, likely your plants will die off, and you will still get some algae. if adjusting the light is not making a signifigant result, back off on the fertilizers a bit. 1 step at a time, adjust 1 thing let it go for a week or two, then make another adjustment if needed. it can take some time to find the magic balance of light and nutrients for a tank. 

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