Aquarium UV Sterilization

Bonus content from the 2011 Marine Fish & Reef USA annual article Turn on the UV.

In putting this article together, I decided to try an experiment on my own reef tank at home. Running a sterilizer on my reef system has made a tremendous difference to the water clarity, and I thought that turning it off might show the slight haze that most reefkeepers don’t even know their systems have. I had no idea how dramatic the difference would be and how much it would make me panic.

To begin, I turned off the sterilizer in the aquarium late one evening. The following day didn’t yield much in the way of results, but the next day I turned on the aquarium lights to find the haze I expected, which was clearly exaggerated by the LED accent lights shining through the water column.

But there was also something else I didn’t expect; on a number of surfaces in the aquarium, a thick, viscous slime was beginning to grow. At first, it had the appearance of coral slime, but the corals in the aquarium weren’t closed up the way I would expect if they were sliming heavily. It was also in places that I wouldn’t have expected slime to be, such as between the fronds of a red macroalgae that I had. The substance seemed to be affecting the water column as well; the protein skimmer cup filled three times that day, which was about 10 times as fast as normal. I suspected that this was some sort of bacterial bloom, but I decided not to do anything that day and watch how it progressed.

The following day, I turned the aquarium on much earlier than normal. The bloom had gotten much worse, and this time I saw a ribbonlike strand from the red algae that indicated that this was indeed a living organism.

I decided that it was time to take action, so I snapped photos quickly and then powered the sterilizer back on. That afternoon I went in with a small siphon hose and manually removed as much of the material as I could, performing a small water change in the process. I repeated this process the following morning and again that afternoon and continued to empty the skimmer cup as necessary. Within two days, the water had cleared significantly. Within four days, the slime bloom had gone away.

Unfortunately, I wasn’t able to secure access to a microscope and positively ID the mystery substance. My best guess is that it was some sort of dinoflagellate bloom that took advantage of the lowered redox potential when the UV was turned off. Defeating it was pretty simple, and it didn’t return when I tried the experiment again a month later.

While I wouldn’t suggest that this type of drastic result commonly occurs once a UV sterilizer is shut off, it is clear that in this case the additional filtration coming from the UV sterilizer was probably helping to keep this messy nuisance bloom at bay. This visual lesson will definitely keep me using sterilizers in any future reef systems I set up!

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Fish · Saltwater Fish