Turning Aragonite into Concrete

A few years ago I decided to redo my 125 gallon FOWLR tank. My goal was to come up with a more attractive aquascape, including adding some more live rock. Over the years I added a bunch of stuff to my sump. A larger skimmer, a small refugium, a powerhead, a filter sock, the list goes on. At this point, the sump was a plumbing and electrical cord nightmare. So I figured while I was at it I would redo the sump as well. As most of you know, aquarium stands are not sump-friendly, so I would need to drain the tank down and pull it away from the wall a bit to remove and redo the sump. I put all the fish in a holding tank, drained the water into several Rubbermaid trash cans, and was ready to start the job.

 

After I got all the live rock out, I would replace the gravel with some new bright-white Aragonite. To my dismay, I started to scoop out the gravel and found it was a solid sheet. All of the gravel was cemented together. It was incredible. I tried to break it apart with my hands, and aside from being very sharp It was very heavy. I wound up using a hammer inside the aquarium, whacking the gravel sheets and breaking them up so I could take out manageable chunks. What’s going on here?

My first thought was that bacteria slime was responsible for welding the gravel grains together. I decided to experiment. I soaked a piece of the gravel rock in bleach overnight. The next morning, nothing was different. It was still a rock. I thought about putting it in a low pH solution like Muriatic acid. But the gravel grains themselves would dissolve and I would likely wind up with a bucket full of Alka-seltzer.

So I checked my reef aquarium for the same condition. It was seeded with gravel from the FOWLR tank. The gravel in there was normal and loose. I mentally went through a list of the difference is these tanks. The reef aquarium had a calcium reactor and the FOWLR didn’t. If anything, I would expect that the calcium reactor could be the cause, but the exact opposite was happening. I drip Kalkwasser in the FOWLR and not the reef, and this was the major difference. I browsed the Internet for posts about this problem, and found many, many aquarists complaining about gravel clumping after starting a limewater drip. I was curious why this is happening. No way can Kalkwasser add as much calcium and carbonates to an aquarium as a calcium reactor. I dove a little deeper into some technical papers and what I found was both the cause and a cure for gravel clumping.

First, a little background. Aragonite is the most popular gravel used in saltwater aquariums. It is made up of mostly calcium carbonates and strontium. It’s created by once living creatures. This gives it a special structure or matrix at the molecular level which causes it to dissolve at a high pH. Aragonite has an equilibrium of 8.2 pH. This means that if it’s submerged in water with a pH lower than 8.2, it will start to dissolve. In reality though, any major buffering effects for a saltwater aquarium really don’t take place until the pH is around 7.7 – 7.8 pH.

Calcite is another gravel sold in the aquarium trade. Commonly called limestone, it is composed of near pure calcium carbonates. However, unlike Aragonite it is created chemically by sedimentation, heat, and pressure. It is mined and crushed into gravel. It’s molecular structure is quite different than Aragonite. As a result, it has an equilibrium of 7.6 pH. It has practically no buffering effects for marine aquariums. Your pH would have to drop to 7.1 before it starts really working. A word to the wise: If you buy calcite gravel for your aquarium, you might as well just use silica sand because neither will buffer anything.

The Mechanics of Clumping

Everyone sets up a new marine aquarium pretty much the same way. You add your gravel to the dry tank, then either add saltwater mixed elsewhere or add fresh water and dump in the salt. In this sterile, chemical soup, a lot of calcium precipitation is happening. All Aragonite gravel grains that are touching one another form a thin layer of calcium carbonate precipitant which is- you guessed it, Calcite. This layer gets thicker as the weeks and months go on, cementing the gravel grains together.

The higher the pH, the more Calcite is formed. Folks who start dripping Kalwasser in their tanks (especially those with low pH to begin with) aggravate the situation. The calcite grows like gangbusters. And Calcite isn’t going to dissolve in your aquarium- the pH is too high for that.

So does Kalkwasser drips causing clumping? No. It is actually the beneficial, higher pH side effect of Kalkwasser that accelerates Calcite formation. But I don’t recommend stopping Kalkwasser to fix this problem because it is one of the best things you can do to maintain any saltwater aquarium. So what’s the fix?

Bacteria slime to the rescue! This is a slippery organic film that eventually covers all surfaces in an aquarium, including all grains of gravel. It blocks the formation of Calcite on the gravel grains. This prevents Calcite from acting as an Aragonite cement. The reason why my reef tank showed no gravel clumping was because 100% of the gravel came from a seeded tank, which was fully “slime-covered”. So the trick to eliminating gravel clumping is to use aged, slime-covered gravel instead of the sterile stuff fresh out of the bag. While most of us use a little “seed” gravel from a mature aquarium to start a new one, it’s pretty hard to find 50 pounds of seeded gravel laying around. But you can easily make your own.

 

 

Depending upon the quantity you need, get a plastic storage bin from the Dollar store or use a trash can and fill it with fresh gravel. If you already have another marine aquarium going, fill the container with used seawater from a water change. If you don’t have another aquarium, your aquarist friends will gladly give you all the used seawater from their water changes.

Place an airstone in the bottom of the container to supply oxygen. Feed the container with anything you have on hand like powdered or wet filter feeding foods, apple juice, etc., anything will work. Let it run for 3 weeks, feeding once a week. It’s also a good idea to stir it it up a bit during this time. At the end of three weeks you will have gravel that I guarantee won’t clump. In addition, it will accelerate the cycle time for your new aquarium setup.