Thank you for your question. Unfortunately you didn’t indicate whether it is leaf-like (brown macro algae) or slimy (coating the sand, glass and/or rock). But generally speaking, when there is an increase in algae growth in an aquarium it is usually a sign of an increase in organics, like phosphates and/or nitrates.
It may be a sign that the bulbs illuminating your tank are getting old and shifting in spectrum (learn more about how often to replace your bulbs). It can also be attributed to the water you are putting into your tank. Definitely check to see if your RO/DI system cartridges are due for replacement.
Now assuming nothing drastic has occurred in your aquarium tank lately (using a new brand of salt, adding several fish, a new or increased feeding regimen), I would begin by double-checking the water quality quality within the tank. Verify all pH, nitrate, phosphate, calcium, alkalinity (etc.) levels are right where they need to be.
If you aren’t currently using a phosphate remover media (like RowaPhos or Phosban) , you might consider trying it for a few months to see if that makes any noticeable changes. Using a fluidized filter, like a Phosban reactor, is a great way to use these types of media in your tank.
If you’ve been a little lax with your water change regimen—most of us are guilty of this from time to time—you may want to do a couple of extra water changes to get back on track.
If your light bulbs are inching towards the end of their recommended lifecycle, consider slowly replacing them (the link above will more thoroughly explain what I mean by “slowly”). As bulbs age, they shift in spectrum and sometimes cause algae to grow faster.
If you make and mix your own water for top-off and water changes, try replacing your reverse osmosis cartridges. Most hobbyists recommend replacing the sediment, carbon and DI cartridges every 6-12 months (depending on usage) and the RO membrane every 2-4 years (more info).
It’s important to note that aquariums go through cycles of maturation, especially during the first 6-12 months. It’s possible your tank is just going through one of these cycles if it’s within that time range.
Additionally, I’ve spoken to many hobbyists who’ve experienced great success using supplements like Prodibio to help clear up algae outbreaks. Their aquariums not only have less nuisance algae growth but the corals have better coloration and growth.
Hopefully these tips will help you resolve your brown algae problem. If you’ve tried most of these solutions already and are still experiencing issues, please contact us again so we can learn a little more about your system.
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