Question & Answer

Our Aquaculture facility rears halibut larvae from hatchling and beyond in large aquaria. The seawater is filtered and ozonated with a contact time of 2 minutes. It is then placed in a holding tank for 8 minutes before neutralization with thiosulfate and finally returned to the aquaria. Using two different ORP meters, the seawater in the aquaria reads +750 mV! Any suggestions what the problem might be?

Ozone is a very powerful oxidant and can elevate ORP (oxidation-reduction potential) values to unsafe levels. Residual ozone present in the water or by-products from the oxidation of bromide to bromate and chloride to chlorate can produce elevated ORP readings.

However, ORP measurements are notoriously misleading and often incorrect. Drain and refill the electrode with the correct filling solution and verify its operation with the following simple procedure:

Dilute an ordinary two ounce bottle of Tincture of Iodine (USP Grade) to 16 ounces (473 mL) with deionized water. After mixing, the fresh solution should measure between +440 and +455 mV. If it does not, repair or replace your electrode.

How does Aluminum affect my marine system? 

Aluminum is acutely toxic to fish {Aquatic Toxicology 31 (1995) 347-356} and invertebrates. Synthetic salts are notorious for containing elevated levels of this element due to impurities found in the chemicals which they are made from. Additionally, many kalkwasser mixes inherently contain substantial amounts of this undesirable component. Aluminum is very difficult to analyze at low levels in the "rich" matrix of seawater. No commercially available kits truly measure the total Aluminum concentration. 

The biological effects of excess levels of Aluminum are unclear. Different lifeforms have different levels of tolerance to this potentially toxic component. If elevated levels are found, one can easily change salt mixes or, if possible use natural seawater which contains very low levels of Aluminum. 

What is the cause of a foul odor within my system? 

Sulfate is present in natural seawater at about 2700 ppm. Some synthetic salts contain elevated levels of this ion. In addition to creating a chemically imbalanced system, it may also cause the following phenomenon to occur. Since sulfate is chemically and physically similar to molybdate, it competes with molybdate assimilation by phytoplankton. If there is an excess of sulfate and/or a deficiency of molybdate, the phytoplankton will convert sulfate to dimethylsulfide causing a foul odor of rotten eggs within your system. Some Aquarists detect this odor in filters or skimmers and attribute it to the decay of waste material. Usually it is an indication of poor water chemistry. We recommend changing salts or judicious addition of a purified molybdenum supplement.

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