Chapter X. Measuring relative levels of DOCs

Cyanobacteria always returned to my tank in the same way. First I would see it on plants, then the bog-wood, then the gravel, then the sides of the tank and equipment. The first sign of it on this occasion was on green hair algae growing on some Java Moss. For a long time I thought it would be a milder infestation than I had seen before. I believed that my now scrupulous gravel cleaning and carbon filtration was bound to reduce it, but history repeated itself. It got worse and worse until it was as bad as it had ever been. To be clear what I mean by that. No matter how much cyano was removed at a water change, within seven days it would grow back to cover all tank surfaces. The only place it didn't grow was on the fish.

It may be indicative of my feelings towards my fish tank during this period, that I can find no photos or videos of the tank. From week 86 (when I first noted that cyano had returned) until week 110, I did weekly 37% water changes with gravel cleaning and watched the inevitable march of the cyano. The only explanation I could think of at the time, was that there was something  in my tap water that encouraged the dominance of cyano, I'll call it X. I believed this because, one of the most depressing aspects of my cyano problem was that I always observed the most vigorous growth within the first 12-24 hours after a water change. It seemed to me that this indicated that the fresh water itself stimulated a surge in cyano growth which then declined, perhaps as a result of the cyano depleting the water of X. This is heresy in terms of aquarium fish-lore. The idea that water changes might be the cause of a cyano problem went against everything I had read on the forums. There was another problem with the theory, the cyanobacteria had first appeared during a period where I was doing bi-weekly water changes, which at the time had seemed insufficient as it had caused a green water algal bloom (see Chapter II). But that was over two years ago. Perhaps my tap water had changed and the initial causes of cyano dominance were no longer the cause. I didn't believe X was phosphate because I had never detected phosphate in my tap water. I didn't think it was nitrate as my tap water had never tested >1ppm. But maybe my tap water was high in DOCs.

I needed a test for DOCs and, although I couldn't find a commercial kit, I did find a method that I think answered my question. I got the idea from this site which describes the various uses potassium permanganate is put to in the aquarium hobby. One of these uses is as a crude test for the relative levels of DOCs. The method is based on the fact that potassium permanganate (KMnO₄) solutions change colour as they oxidise DOCs. A good example of the colour changes observed can be found here. So I took a 10 ml sample of my tank/tap water and added five drops of a KMnO₄ stock solution.

Both samples were the same colour to begin with.

Here is a quote from the skeptical aquarist link above.

You can see the reaction happening, as the magenta pink color of unreacted KMnO₄ oxidizes first to a rosy tea color, then to amber and brown; the time it takes to spend itself depends on the concentration of dissolved organic matter. In fact a rough-and-ready field test for dissolved organics measures the time it takes for KMnO₄ to completely oxidize in a water sample.

4 Hours

As you can see, the tank water sample had changed to a rosy tea colour by 21 hours.

21 hours

The tap water sample had remained unchanged, which told me that my tap water was lower in DOCs than my tank. So a water change would be expected to reduce the level of DOCs in my tank, contradicting the idea that X = DOCs. This situation reminded me of Chapter IV when I didn't know if my tap water was high in nitrates and phosphates. Back then, even though I had measured that nitrates and phosphates were low, I went ahead and spent weeks lowering them further. This time I decided a complete change in tank water chemistry was the only way I would be rid of cyano.