Chapter IV. Cyanobacteria control by PO4 limitation

The problem I had was, I had no idea what the levels of phosphate and nitrate were in either my fishtank or my tap water. Improving circulation hadn't helped. But, I couldn't be sure that by frequently changing the water and vacuuming the gravel, that I was reducing NO3/PO4. So in week 30 I bought myself Salifert test kits for nitrate and phosphate. My tap water tested ~1ppm for NO3 and I got no blue colour at all with the PO4 test which according to the manufacturers means less than 0.03 ppm (<0.03 ppm).  My tank was 2.5-5 ppm for NO3 and <0.03 ppm for PO4. I tried the tests on my unheated goldfish tank in the front room and got 25-50 ppm  NO3 and 1 ppm for PO4 so I think the kits were working. The levels I found in the cyano tank were low from what I had read, even reef keepers might have been happy with them. It seemed that, if excess nutrients had been the cause of my cyano outbreak, then I had removed the cause. But the cyano persisted, in fact it thrived. I decided that since I was doing water changes like I was overstocked, I may as well be overstocked, so I put in five cardinal Tetra and hit the internet, big time.

The strategy that caught my eye was all about controlling algae and cyano in planted freshwater tanks by phosphate limitation. The idea was that if you stimulated plant growth enough by supplying nitrate and potassium (K) and CO2, but no phosphate, then the plants and algae would use up all the phosphate and the cyano would die. There was no way I was getting into CO2, not in a low light tank, so I started adding K2SO4 so as to achieve 15 ppm K (the four mineral water companys around me all say their bottled water contains 1-2 ppm K) and trace elements. I also monitored NO3 levels. These remained at 2.5-5 ppm for the first five weeks, but then dropped so I started dosing KNO3 to keep levels in a similar range. Eight weeks of nutrient dosing, water changing and gravel vacuuming followed, the cardinals grew strong and wiley. It was during this period that I replaced my arch-like tank ornament with a piece of bogwood. I had read that the tannins from bogwood reduced pH. Some claim cyano prefer alkaline conditions which may be relevant as the pH of my water is 7.6-8.0. I had also read that tannins were toxic to cyano. Here is a still from a video shot on Sunday 3rd April 2011 (week 37).

The plants have grown nicely but the thing that makes me think the nutrients made a difference is that the Ellodea have changed, they have broader and longer leaves than previously. Unfortunately the cyano was also doing well. Not exactly the behaviour of an organism that was starved of its food supply. Note the lilaeopsis has gone, plants on the bottom are difficult to clean and tend to die. I have no memory of the small broad leaved plant I've planted in its place.

This is the day after a water change. I siphoned a load of cyano off the Ellodea but didn't have time to do the bogwood before my 20 liter bucket filled up. The bogwood had no effect on the cyano I could see, if anything it seemed to grow more lushly on the bogwood. Maybe my piece of bogwood was so old that it had leached all its tannins out. It didn't seem to discolour the water but I was doing frequent water changes. The gravel surface looks OK, the cyano on it may have grown back overnight, but notice the filth below. Looking back now I can see that my hours of gravel vacuuming had not prevented the build up of mulm, otherwise know as fish s**t. But who cares? There should be some mulm in the gravel, it's part of the Nitrogen cycle. It gets broken down by bacteria into ammonia and NO3 which can be taken up by plants. Setting up a fish tank is like establishing an ecosystem and it seemed to me that some decaying matter was to be expected. So why was my ecosystem increasingly dominated by cyano? Confused, I once again suckled from the teat of the internet. In desperation I decided it was time to seek medical help.