Mr. Saltwater Tank

Terrible Advice Tuesdays: What’s Really Needed to Grow Coraline Algae

Terrible Advice Tuesdays: Coraline algae will only grow under metal halide lights.

The rest of the story: Those metal halide manufacturers must really be grasping for straws these days!

Algae needs light and nutrients to grow. Coraline algae also benefits from higher calcium and magnesium levels. And some tanks, despite having light, nutrients, and correct calcium and magnesium levels still don’t grow coraline algae. Is it the lack of metal halide lights? Negative.

I haven’t had a metal halide light over any of my tanks, or any of my dozen of client tanks, in years and I’ve had plenty of coraline algae growth. Heck, in one of my quarantine tanks is lit with a fluorescent shop light and there is coraline algae growing in it!

Forget needing metal halide lights to grow coraline algae. That’s a joke and is truly laughable.

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Comments for this article (23)

  • David Moulton says:

    While I agree the metal halide story is bogus I think you should follow up with useful suggestions as to why coraline algae may not be growing if all expected factors (i.e. lighting, nutrients) are present. This would make for a more helpful post.

  • Matt says:

    I had MH for years without any growth of CA. I did a tank refresh last year and moved to AI LEds and I have had explosive CA growth. While I know its more than the lights I was always pissed at being told I needed to spend ridiculous money on MH bulb replacements for years with no CA payback.

    I also have kept my nutrients in check and used better quality salts with H2O changes more frequent.

    Careful what you wish for you may just have CA all over the place with yet another thing to manage and to date- there is no CA removal that’s automated!

  • Jeff says:

    I really don’t understand this obsession with coraline. It does what it does, and really has nothing to do with the health of your animals. We posted a picture of our tank to a forum and a guy asked what I did to get so much of it. I told him I really didnt do anything. It just did its thing and that I wouldnt worry about it and I think he was offended

  • Bryan says:

    In my opinion, the only real benefit you gain from coraline is that it makes your liverock look much better. Other than that it is just one more reason that I have to clean my glass regularly.

  • Dale says:

    I have had 2 tanks now under the same compact florescent light and both tsnks have had a good amount of coraline algae.

  • Phil says:


    I have had CA growing like a weed under T5s.

  • JasPR says:

    well, yes, the observation is likely incorrect– but I feel compiled to mention— some lights WILL distort , or put better- ‘favor’ some species over others. I mention this once before as was surprised the idea was pushed back on? certain array of LEDs for instance will absolutely explode undesirable algae proliferation IF the other requirements for growth are there. certain companies ( un-named out of respect) have changed their array as the older units were exploding green algae growth cycles. in another example, IF you have organics and nitrogenous waste in the water and use excess lighting, expect red slime algae forms.
    And many report that was their MD lamps age the mix and nature of the algae in their tanks changes. In the end, it is a box of water with life inside and we attempt to harness actual natural life cycles and systems in order to keep it alive. not very ‘natural’. on the other hand, one biological concept– competitive exclusion- is VERY MUCH alive and real in our glass boxes. JasPR

  • James says:

    Hi Mark, what in your opinion is the best bio pellet reactor.

  • Patrick says:

    I’ve had my tank up and running for 14 years, start it as a FOLWR tank, in the beginning I purchase some live rock with solid purple corline, sense with additions of new coral, time and base rock my tank is pretty much solid purple, and pink. One help in getting your corline to grow is to introduce it to your tank. Like all algea it has its own life cycle before it is the dominate form of algea in your tank. Corline, and it’s types thrive in many different lighting senareo, there is low light, and high light varieties. Good calcium and magnesium levels as Mark has said, seeding your tank with it, and with all things in this hobby time……..

  • Travis says:

    This is a breath of fresh air, but i have another question and anyone please feel free to answer…. I have heard recently that ppl are leaving the led lighting and going back to t5 and metal halide lights because they had better results with sps corals and clams with those. Anyone had any similar experience?

  • Carl says:

    Ok, coraline , If your rocks are covered in it, doesn’t that clog the porous holes that the beneficial bacteria live in???

  • JasPR says:

    Carl, I find it helpful to think of bacteria beneficial forms as not millions of individual cells but actually as ONE organism in balance with the maximum output of nutrient put out by all ammonia producing species. This massive organism is like a rheostat in that it is regulating and self adjusting ( either in numbers or by cell membrane expansion. So if you build it, they will come- regardless. As for denitrification,minimal is best

  • Dan says:

    Travis, I get great color and growth out of my LEDs for SPS and LPS. The trick was adjusting the intensity of my LEDs(4 Ecotech Radions) I bleached a lot of frags before reducing to 50% then slowing bringing up to 65%. I would only buy an LED that has adjustable intensity.

  • Patrick says:

    As with all things concerning from our seas, opinions are as varied as its inhabitants. I’ve read the same thing about corline stopping up the lightsvin your rock. As long as you have an effective biological filter (rock), and any surface in your aquarium bacteria grow. So much data can mislead even those of us who have been aquarist for years. You have to read every thing, then base what you have learned by imperial data (what’s working for you) There are as many misnomers, as their is good hard facts, and facts concerning reef aquaria. One of the more fasinating aspects of our hobby it’s always a learning science. I started late in the 80’s and the amount of information, and change I have witnessed is remarkable. I’m glad their are guys like Mark, and others on the internet, who put it out their for us to chew on…..

  • JasPR says:


  • Darren says:

    Taken a look at any one that runs a light over their sump. Most will have tons of coraline. Mine sure does with just a PC 6500k bulb.

  • Travis says:

    Dan … Thanks so much!!! I was thinking i was throwing money out the window!!!

  • Lee says:

    Dont forget that there are different forms of coraline, different colors that have different lighting requirements… my tank isnt purple, but I have lots of coraline…

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  • Will says:

    My first tank came and this is all I was concerned with. I wish I had listened when everyone said “just relax, it will come.” In my experience, the last thing you will kill is your coraline in a reef tank. Unless your params are super out of whack, you’ll get this crap everywhere. I just started a new tank 2 months ago. It only has 2 160w Maxspect Razors hung about 8 inches off the water, and I already have coraline growth. Sure its pretty to look at, but it make maintenance awful. You wont like it for very long.

    Travis, I’ve read a lot about people switching “back” to t5 or MH, but I honestly think these are just old timers stuck on MHs (haven’t really tried LEDs). The other thing to keep in mind is there are a ton of low-quality (cheap) LEDs that just don’t have enough watts to penetrate the water. I suspect many people have tried these, failed, and said “Well LEDS are useless!”

    If you check with a PAR meter, you may be surprised how much PAR these high-quality LEDs throw.

  • Travis says:

    Will- thank you sir .what are the leds to use ?stuff like radions i take it …. I know all are good quality but is there a manufacture that stands out from the others?

  • Will says:

    I actually am running 2 160w maxspect razors over 120 gallon (24″ deep, lights about 8 inches off the water). I did have the P series modules before and decided to upgrade (didn’t like the fans in those personally). From the PAR examples I’ve seen, radions are better. Maxspects are cheaper.

  • Beetle Bailey says:

    The light story is so true ,I now run LED’s and the CA growth is the same as with the MH lighting, does any one scuba dive, have a look at a real reef , you dont see all the rocks covered in CA, while I agree it looks great but I think thats the only benefit of having lots of pink CA in you tank,having said that if you do have good growth it is sign that your water parameters are pretty good

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