Reef Tank Calcium and Alkalinity

Reef aquarium alkalinity and calcium levels.

Q. About two years ago I set up a reef tank using a standard 55-gallon tank. I built a biofilter from a 10-gallon tank and a 4-gallon square plastic pail equipped with a drip plate and filled with PVC media. A Rio 2100 sump pump circulates about 250 gallons per hour through the tank, and the nozzle on the water return is necked-down to give good, if not vigorous, water flow throughout the tank. A portion of this return water (about 1 gallon per minute) is diverted through an external (3 x 3 x 20 inches) air-driven protein skimmer. It works well in that the collection cup accumulates brown scum. Initially, I had four fluorescent 4-foot tubes on this tank, but have since upgraded to two VHO 110-watt 50/50 lamps, which did not seem to make any difference.

The tank has about a ¼ to ½ inch of coral gravel on the bottom, and 55 pounds of live rock. It is stocked with two small sebae clowns, a pygmy angel, a pink and yellow basslet, a mated pair of peppermint shrimp, a cowry snail, an umbrella-shaped snail, and a pencil urchin, all of which must have come with the live rock. Among the other invertebrates that are doing well are two Atlantic and two rock anemones and a mushroom-shaped anemone.

The organisms that haven’t shown any growth, and seem to be doing worse, are the bubble coral and several soft corals. I’ve tried moving these around to areas of different light levels and water flow rates without noticeable improvement.

In regard to maintenance, I initially did 10-percent water changes once per month, and have stepped up to 10 percent every two to three weeks. I use an Aquarium Pharmaceuticals de-ionizing unit immediately after a two-stage drinking water filter. I rinse the biomedia in saltwater every six months to remove what little crud has accumulated. I feed the fish one to two times a week. I also add calcium supplements, strontium supplements and protein supplements once a week in the recommended dosages. Recently, thinking my soft corals were starving, I started offering store-bought invertebrate food, but there’s been no noticeable change in the corals.

I’m not a high-tech aquarist (I don’t have thousands of dollars worth of electronic gizmos monitoring the redox potential, and so on), although I do monitor the water periodically with a test kit. The pH is steady at around 8.0 to 8.2, and nitrite, nitrate and ammonia levels are not present in measurable concentrations (less than 1 part per million). I add very slight amounts of kalkwasser to the evaporation replacement water.

So what am I doing wrong? Too many fish? Too much light? Not enough light? Nutrition? It pains me to see these invertebrates (apparently) struggling to survive in my tank, and sometimes I consider just changing over to fish entirely, but I thought I would give this reef tank one last chance.

A. After reading your letter I was a bit mystified at first as to what the problem may be. It’s always difficult to diagnose a problem from a distance, and doubly so when the communication can only be one way. There are a number of things I would like to have known in addition to that which you provided.

After rereading your letter a few times I think I can offer some suggestions. It appears that any organism that requires calcium carbonate is not doing too well in your system, with the exception of that rogue leather coral. After looking at the water quality parameters and maintenance habits, there are two glaring omissions: alkalinity and calcium levels.

As I explained in the March 1997 issue of AFI, corals require calcium and bicarbonates to be in the water in adequate amounts in order to calcify and build their skeletons. This is also true for stony corals. But many aquarists seem to forget that so-called “soft” corals also require these elements. Embedded in their gelatinous tissues are small calcium carbonate particles known as spicules that help the coral maintain its shape and structure. Unless there is sufficient calcium and bicarbonate in the water, they will not be able to build their spicules and they will therefore not grow.

The small amounts of kalkwasser you are using, your weekly calcium dosages, and even your water changes are most likely inadequate to maintain calcium and alkalinity values within seawater norms (400 to 450 milligrams per liter calcium, 2.3 to 3.0 milliequivalents per liter alkalinity). I would suggest investing in a couple of test kits to measure these parameters, and greatly increasing the amount of kalkwasser powder you are using to at least two level teaspoons per gallon of makeup water. This should be slowly dripped into the tank, preferably in the morning when the pH is the lowest.

If you find that the levels are very low you can use one of the new two-part solutions on the market that boost both alkalinity and calcium without increasing sodium or chloride levels (B-Ionic from ESV and C-Solution from Two Little Fishies come to mind) to bump the levels back up quickly. Then use kalkwasser to maintain them.

Although you may not be a “high-tech” aquarist I think you should invest $100 in a pH monitor. This will give you constant feedback on the pH and help you monitor the effects of kalkwasser additions. Also, wide swings in pH over the course of a day are sometimes symptomatic of low alkalinity, and a meter can tell you this much easier than using a test kit several times a day.

Of course, another thing I would suggest is removing the biomedia in your filter. Simply stated, you don’t need it with the live rock you already have. I would also either build a new skimmer or purchase a larger commercial one. Almost any skimmer design will pull out “brown scum,” but that does not necessarily mean it is filtering your tank efficiently, and I question whether your skimmer is.

You might want to consider replacing the coral gravel with finer coral sand, or hydro-vacuum the gravel regularly to remove accumulated debris. I also think you need to upgrade the water motion in your aquarium by using an additional powerhead in the tank. Finally, while you don’t have too many fish, I would watch the angel closely to see of it is picking on the corals.

Article Categories:
Fish · Reef Tanks