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Testing for Bacteria in the Reef Tank

Keeping a healthy reef aquarium.

Keeping a healthy reef aquarium.

Bacteria make up one of the largest groups of organisms on earth. They are all around us — in the ground, in the air, and in both fresh and saltwater. Bacteria are the organisms that convert potentially harmful nitrogen compounds into harmless nitrogen, enabling reef hobbyists to maintain invertebrates that would otherwise perish as the compounds increased in concentrations in our freshwater and saltwater fish’s aquarium water.

The bacteria associated with denitrification are helpful, but not all bacteria found in a reef aquarium are. One aquaculture study found Pseudomonas, Oceanospirillum, Marinobacter, Paracoccus, Erythrobacter, Vibrionaceae, Vibrio and Aeromonas in a recirculating saltwater system. Some of these bacteria are harmless, but others are potentially harmful in high densities.

Bacteria have been implicated in RTN (rapid tissue necrosis). While the tie remains speculative, there is some indirect support in scientific literature. The mucus of corals is a rich breeding ground for bacteria, and in stressful conditions corals produce prodigious amounts of mucus. Bacteria may not precipitate the initial outbreak of RTN, but an accumulation of bacteria may increase the likelihood of RTN spreading to other animals.

Until recently, determining the density of bacteria was a time consuming task reserved for laboratories with special equipment. Now there is a test that enables a reef hobbyist to determine the level of bacteria in a reef aquarium quickly and relatively inexpensively. The test is the BART Heterotrophic Aerobic Bacteria (HAB) test created by Droycon Bioconcepts of Canada. BART stands for Biological Activity Reaction Test. A small quantity of aquarium water is added to a vial of methylene blue dye. The water initially turns blue. As the aerobic bacteria consume the methylene blue, the blue color turns to an off white color (Photo 1). The time it takes for the blue color to disappear (a process called bleaching or reaction time) is an indication of the density of bacteria. A complete explanation of the process can be found at the Droycon Bioconcepts website,

Bob Stark of ESV first brought the test to my attention a year ago, and since then has enlisted the help of hobbyists to test their reef aquariums. In addition, I’ve tested the water of coral reefs in the Caribbean and Indo-Pacific so that we might compare reef aquarium bacteria levels to those found on the natural reef. While we are still gathering data, indications suggest that the HAB test may be valuable in monitoring the health of a reef aquarium, and for the first time provide an objective measure of the relative conditions of different reef aquariums. More work needs to be done, but preliminary data suggest that in general natural coral reefs have lower bacteria levels than most reef aquariums. More importantly, healthy reef aquariums seem to have a lower bacteria level than unhealthy reef aquariums.

Newly made artificial saltwater has a reaction time well in excess of 30 hours. In other words, the vial water remains blue for over a day. Natural reefs vary, but for the most part have a reaction time of 14 hours or more. The areas close to shore where there is a great deal of stirred sediment are an exception. In these conditions, the reaction may occur in less than 12 hours. This is consistent with scientific literature that suggests that sediment is rich in bacteria. In areas where the sediment is suspended, one would expect a higher level of bacteria. (Many hobbyists have reported episodes of RTN after stirring up a reef aquarium’s sediment. It is quite possible that stirring the sediment stresses corals and also releases bacteria into the water column. The excess mucus produced by stressed corals combined with higher levels of bacteria may explain the outbreak of RTN.)

The healthiest reef aquariums tested have had reaction times equal to natural coral reefs. Well-maintained, thriving aquariums have been in the 12+ hour range. However, aquariums that appear to be suffering from “old aquarium” syndrome have had reaction times considerably less than healthy aquariums. Some aquariums have had a reaction time of as little as six hours.

The difference between eight and 14 hours may not seem like a lot. However, bacteria grow exponentially. The eight hour aquarium may have over twice the bacteria density as the 14 hour aquarium (Figure 1). Even a healthy reef aquarium has more than 40 times the bacteria as freshly made artificial saltwater. Hobbyists have generally observed a dramatic improvement in the condition of a reef aquarium after doing a large water change. We’ve speculated about the possible reasons why a water change might improve the health of a reef aquarium. To that list, we should add lowering the bacteria count of the aquarium.

The testing completed thus far raises questions about how we might lower bacteria levels in aging aquariums. Aquariums that have utilized ozone or UV sterilization generally have lower bacteria counts. After finding that my oldest reef aquarium had a reaction time of only eight hours, I added a small ozone unit. Over a two month time, reaction times increased to 12 hours. Subjectively, I felt the aquarium looked better, and other hobbyists seemed to agree.

Using the BART-HAB test offers the possibility of a test that may warn us of potential problems in a reef aquarium. If bacteria levels increase in a reef aquarium, the aquarium may be more prone to problems, including RTN. If the test provides an early warning of a problem, the investment in the test may be well worth it. I’ve begun periodically testing both of my aquariums.

The test may be useful in evaluating the use of ozone, UV sterilizers and activated carbon. The hobby has drifted away from the use of these chemical filtration methods in part because we had no way to evaluate their efficacy. Measuring bacteria levels may be a useful tool in evaluating chemical filtration methods. Ozone, UV sterilization and frequently changed activated carbon have all proved useful in lowering bacteria levels. The experience with carbon is instructive. The addition of carbon initially reduces bacteria levels. However, if the carbon is left in the system too long, bacteria levels increase to previous levels. Protein skimming has not proved effective in lowering bacteria levels.

One might argue that ORP levels are just as useful as measuring bacteria levels. I found that with the use of ozone, ORP levels increased as bacteria levels decreased. However, ORP seems to be an imperfect measure. The mature aquarium I mentioned had always had an ORP level in the 300 to 350 mV range, a level considered normal for a reef aquarium. During this time my new aquarium had an ORP of only 200 mV and yet a HAB reaction time of nearly 12 hours. Using ORP, one would conclude that my older aquarium was healthier than my new aquarium. However, subjectively I felt the new aquarium looked much more healthy, and the HAB test confirmed it.

This is an entirely new uncharted area for the hobby, but an encouraging one. We may have a very useful tool, but we need to gather more data. I hope all hobbyists will conduct at least one test on their reef aquariums and begin comparing notes on bulletin boards and mailing lists. If we can collect enough data, particularly comparing healthy and unhealthy reef aquariums, we’ll be able to tell how useful the test is. There are at least two ways to purchase the test. One is through Hach Company at The product number is 24904-09. The only downside to ordering through Hach is that they sell the test only in test packs of nine for $80. This might be appropriate for clubs, but nine is really too many tests for most hobbyists. ESV will be selling individual tests and also including useful information for hobbyists. Check ESV’s web site at for availability and prices.

Further progress in reefkeeping requires that we pool our experiences and knowledge. Using the BART-HAB test to learn more about the impact of bacteria levels in a reef aquarium may be a giant step in the right direction. I hope that clubs and individual hobbyists will share their experiences with the BART-HAB test.

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Article Categories:
Fish · Reef Tanks