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How to choose good live rock

The first thing we should understand is that live rock is not actually alive, it is rock, sometimes coral skeletons or fossils of such, but it is not actually alive. This title refers to the bacteria, and other life that dwells on and within the rock. The positive affect of these on your tank is what you’re looking to accomplish. Not all live rock is created equal though, and an aquarist should know just what to look for when buying.

How much live rock do I need?

Because live rock can be expensive, many people are concerned about just how many pounds of it they need in their tank. 1 to 1.5 pounds per gallon is a good estimate.  You can do more or less depending on the quality of the rock, the livestock in the tank, and the type of equipment you have to protein skim/filter your tank.

Look at the live rock

  • First you are looking for a rock that is aesthetically pleasing to you.  There’s a great deal of artistry in aquascaping beyond the placement of corals.  Find a piece of rock that you like the look of.
  • Look for a piece with plenty of holes and crevices in it.  These are not only good hiding places but mean greater surface area for bacterial colonization.
  • See what’s visible on the rock. Is there anything growing on it and if so, is it something desirable? For example, two of my favorite corals are a colony of button polyps and another of mushrooms, neither of which I payed for, they were hitchhikers on the live rock I purchased.  They were small at the time, but with the addition of light and nutrients they are both now quite beautiful. Pest anemone, like the aiptasia, will also be found on the rock, so look out.  Crabs, both beneficial and harmful may be easily spotted at times, but may hide well others. In the picture to the right there are button polyps, a vermetid snail, and coralline algae all visible, and all come free on the rock.
  • Inspect the color of the rock. Some coloration besides gray, is to be expected and may indicate something beneficial.  for example a pink or reddish color could be an indication of coralline algae, which is a good thing.  Black spots could indicate something dead and thus likely to have negative impact on the tank. This doesn’t mean you won’t buy the rock, but it may mean some extra work for you when you get it home, which we will discuss later.

Feel the live rock

There are two things you are looking to discover when you pick up the rock:

  • How heavy the rock is.  There are two very important reasons to find light weight rock.
    • It’s sold by the pound so lighter rock costs you less.
    • Lighter rock means it is porous. As previously mentioned this means nooks and holes and that equals greater surface area.
  • Is the rock slimy? It shouldn’t be. If it is, that may represent something dying on the surface, or a type of bacteria that isn’t the beneficial kind.  Again, you can still buy the rock if you like, but it will require some work.
  • The surface should be rough.  Some rocks are worn smooth by the action of water upon them.  This is typical of rocks in a river. It could very well be that the rock is from the ocean, or it could be someone is trying to pass off a river rock to make a buck.  A smooth rock does not provide as much surface area for beneficial bacteria to grow.  Avoid overly smooth rocks.

Smell the live rock

Yes, you read that correctly, smell the rock.  Pick it up, put it close to your nose, and take a gentle sniff.  The rock should smell good, perhaps a little bit like the ocean, or have very little scent at all.  If the rock has a strong odor of fish, or smells rotten, then that’s a sign the rock isn’t cured yet.  It doesn’t mean you can’t buy it, if it is the best looking rock you’ve ever seen, but you will have to do some extra work before putting it in your tank.

Prepare your live rock

In the above there were several mentions of “extra work” that might need to be done to live rock if it doesn’t meat certain conditions right from the shop. This is known as curing. For the most part, a quality fish shop will only sell previously cured rock which should be ready for your tank.  Sometimes though, this isn’t the case.

When the rock is taken from the ocean, and transported across the country, there will be a certain amount of organisms that do not make the trip.  These dead organisms are what can cause a slimy feeling or a bad smell to the rock.

There are just a few easy steps to get your live rock ready for use:

  • Lightly scrub the surface of the rock with a stiff-bristled brush. This removes the dead and decaying organisms.
  • Rinse the rock off in salt water.  Using fresh water, or worse chlorinated water, will kill the organisms that happily made the trip alive and render your prize just another rock!
  • Place the rock in a container of salt water with a heater and a circulation pump (no light is required to prevent algae from growing).  This can be an aquarium with no animals in it, or it can be a simple plastic tub.  Allow the rock to remain there for about a month, after which time it will be ready to place in your tank.


Once you place the rock in your tank, there will be a change in the chemistry of the water.  Even the best live rock will cause some alteration to the bio-load.  Simply monitor the water, and during the first couple of months do regular water changes, about 5% – 10% a week should suffice.

Now you’re ready to go and buy live rock feeling confident you know what to look for.