Algae Frag Tank

Tips For Keeping a Frag Tank Algae Free

Many reef keepers, including myself, utilize frag tanks to house corals propagated from colonies in a main display tank or to park corals that can’t fit in the display (some of us are hoarders!). They can also be used to quarantine corals but this type of setup requires a separate system on top of the one used for a display.

An alternate route is to plumb a frag tank into the same system used for the display, a much cheaper and easier to maintain solution since no extra equipment such as a skimmer or calcium reactor are required. If you opt for this setup you should consider a few things to avoid issues with problematic algae. Yes, even if your display is algae free you can have algae outbreaks in a frag tank connected to the display. Keep in mind that these tips also apply to just stand alone frag tanks and displays.

Go With the Flow

Strong circulation makes it tough for algae to take hold in a frag tank since it keeps detritus from settling on the bottom, preventing the build-up of excess nutrients that can spur on algae growth. For this reason I find it beneficial to go bare bottom in a frag tank versus sand since high flow can cause a sand storm. Go big or go home when it comes to flow.

One option is to have a lot of flow concentrated at the bottom of a tank. When I initially setup my 75 gallon frag tank the frag racks were four inches off the bottom, allowing me to crank water underneath the racks with a re-circulating pump. This worked well but the grow-out colonies started to run out of room (the tank is only twelve inches tall) so I had to pull the racks and place the corals on the bottom.

Besides having more room to grow, the corals also benefited by having more direct flow. Why didn’t I do this in the first place? Well, I was worried the tiles with the grow-out colonies would get pushed around by the current. Ultimately, this wasn’t an issue with the more mature colonies since they were heavier.

Clean-up Crew

A proper clean-up crew will also go a long way in keeping algae at bay. Both Trochus and Astrea snails do a great job munching on algae and fish such as Tangs or Rabbitfish will help as well. Don’t skimp on these critters. I have a ton of snails in my tank plus a Foxface Rabbitfish, Yellow Tang, Kole Tang and Sailfin Tang. With Tangs it is best to put similarly shaped ones in all together to avoid fights over territory.

Both Scarlet and Blue Leg Hermit crabs are good herbivores and are excellent additions to a frag tank. Emerald Crabs can help if bubble algae starts to pop-up and Peppermint Shrimp will aid the fight against Aptasia.

Avoid Introducing Phosphate

Can a frag rack cause algae to pop-up? Some believe egg crate used to hold frag plugs can leach phosphate, thus spurring on algae growth. And there is anecdotal evidence that algae is more likely to grow on white egg crate versus black egg crate. These are interesting theories but I don’t believe there is any scientific evidence to back them up. Nonetheless, I do use acrylic frag racks since they supposedly will not leach phosphates.

Another thing I do is “soak” my frag racks in my sump for a week or so to allow some bacteria to colonize the new surfaces.  My thinking here is that algae will be less prone to grow on something with bacteria versus a new surface without bacteria.  Just a theory of mine. Finally, fish food contains phosphate so be careful to not overfeed your fish.


Following the tips above should help to keep a frag tank algae free and hopefully promote the growth of purple coralline algae, a big plus since this type of algae will make it tough for problematic algae to take hold in the future. Purple is good, brown and green are bad 🙂

Additional Resources

If you would like some help with a new tank build, including help designing a custom aquarium, or help re-configuring your current setup then you can visit this page for more information. And if you are looking to add some equipment, I do sell GHL, Pax Bellum, Reef Octopus Calcium and Kalk Reactors and Royal Exclusiv products, including Dreamboxes, which is the equipment I use and recommend. I also sell Reef Brite metal halide and LED fixtures as well as Maxspect & IceCap Gyres.

As for additional insights and information, please explore my many other reef tank and SPS related articles as well as my YouTube channel. For an even deeper dive into reef tank care you can check out my Reef Keeping Master Class. This online course is an immersive and one of a kind educational tool designed to help reef aquarium hobbyists build and maintain a beautiful SPS reef tank. The course is a series of video presentations with some supplemental video from my YouTube channel. There are also quizzes to help students retain and understand the information presented in the course.

Need some frags…..I can help with that as well 🙂 Please visit my SPS Frag store to see what is available.

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