Hi toc,
Well, since you are under two months with the tank I guess it may be fishless cycling, although I can't know that without looking at your other threads. That would certainly supply the -other- of the critical ingredients that brown algae is looking for, lol. (But regardless of your type or status of cycling the following comments should apply.)
That's because the only two things that algae spores need are Ammonia and Light! Algae spores (a dormant state that allows the single celled plant to last a very long time without having the right conditions) are extremely widespread in natural fresh water bodies all over our planet, much like the bacteria that appear in our tanks. So the spores are nearly always there and waiting.
The "right conditions" for the spores to hatch out are very minimal. They need a tiny bit of ammonia (considerably less than our ammonia test kits can measure, so other than the first couple of weeks of tap water, our tanks provide enough even when ammonia measures zero ppm.) But most of all they need light, in conjunction with that trace ammonia, and even though they need considerably less light than vascular plants (our non-algae plants) they do respond differently to different lengths of light exposure.
For us freshwater tank keepers, there are two critical starting points in our algae prevention: either minimal light (if there are no plants) or 4 hours (if there are vascular plants present.) There is also the "power" (actually the brightness) of the light. So, just like we'd think, both quantity (of photons) and exposure time (hours) matter. Control of the tiny ammonia amounts is also a possibility but I'll mention that later.
First, always determine and note your lighting power as a baseline statistic of your tank in the aquarium notebook for that tank. If you have T8 (8/8ths = inch diameter) or T12 (12/8ths = 1.5 inch diameter) then an old rule of thumb tells you that less than 2 watts per US gallon will work well for dark-leaved "low-light" plants and above 2w/g will push you into "high-tech" planted tank techniques, perhaps. If you have T5 tubes (5/8ths = 5/8 inch diameter) or compact fluorescent or incandescent or metal halides then the rule of thumb changes and you can't use the same wattage as a rule of thumb. (I realize you don't have plants but all this is helpful background, so hang with me I hope
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OK, so now you know how much attention we pay to the power and hours of our lights! As a beginner with brown algae and no plants, your most powerful tool is light hours. Plants and humans are the only things that have serious need of a lot of light. The fish see better than we do and are used to water at night or in shadow, so they adapt much better than we think to darker waters. (-Most of our fish -do- appreciate a reasonable attempt at night/day simulation though, especially since science now shows that true blackness at night is a healthy thing, for both humans and fish!) Anyway, your solution (which of course I could have said in one sentence) is to not leave that light on nearly so much! In a tank without plants you should only turn it on when viewing and if you feel the need to leave it on for occasional viewing, you should limit it initially to the 4 to 6 hour range and see (over weeks) how it changes your algae situation. (of course in your situation the wattage/type is still important too)
In tanks with live plants, we can start at 4 hours and and gradually increase an hour or half hour at a time until we get algae, watching for at least a week or two in between changes (yes, its that slow to know, I think.) Also, its usually a more advance topic (meaning most beginners should focus on watts / US gallon and especially number of hours of lights on, at first) but using a chemical adsorbent like Purigen in a post-biological filtration stage can reduce trace ammonia enough to also combat algae, assuming you can afford large enough filtration beds and equipment.
Again, bottom line for you is simple, figure out brightness and plan a shorter "photoperiod" (single word for number of hours lights stay on.) You might also note that brown algae (diatoms) likes low-circulation areas, so will get thicker in tank corners and on leaves and gravel and other places where it is slightly out of the main flow. All algae is triggered by light and ammonia, but each of the many algae species may have other factors that are important when you are battling an outbreak.
~~waterdrop~~ (morning coffee mode
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