I just thought I’d share the results of my experimentation with nutrient ratios in relation to cyanobacteria. I believe the ratio of NO₃ and PO₄ may be important.
To start with, I was dosing TNC Complete at the “double-triple” dose as recommended by Aquarium Gardens. Roughly speaking this results in about 40 ppm/week of NO₃ and about 3.7 ppm/week of PO₄. In other words, this is very rich in both NO₃ and PO₄ and not far off a 10:1 ratio. With this dosage I was, not surprisingly, getting good plant growth - and I was getting no Cyanobacteria at all.
About eight weeks ago, I decided to stop using TNC Complete and go with DIY salts instead, because I’d read up about red plants being redder when NO₃ is restricted, and I wanted to make my Rotala wallichii go a bit pinker. So I started using KNO₃, KH₂PO₄ and K₂SO₄ to give me 15 ppm/week of NO₃ and 3 ppm/week of PO₄ - so quite a big reduction in NO₃ and a small reduction in PO₄.
Under this regime I was still getting good plant growth, but I started noticing small pockets of Cyanobacteria, particularly near the tips of stem plants in the brightest parts of the tank.. When I say “small pockets” I mean a few square millimetres that never really got any momentum going. I also got green dust algae growing on the front glass very rapidly indeed, which I hadn’t had before.
You often hear that Cyanobacteria is caused by ”low nitrates”, but I wondered if it’s actually the ratio between nitrate and phosphate that is the controlling factor. So about three weeks ago I decided to alter my dosing. I left the NO₃ the same at 15 ppm/week, but I lowered the PO₄ to 2.0 ppm/week. Plant growth is still okay, but the small pockets of Cyanobacteria have packed their bags completely. Also, the green dust is now much slower.
So it appears that the reduction of PO₄ has booted out the Cyanobacteria. But it can’t be the level of PO₄ that was the problem, because originally under the “double-triple” dose of TNC Complete I had a much higher level of PO₄ but no Cyanobacteria. So I think it must be the ratio of NO₃ and PO₄ that created conditions that made it just about possible for Cyanobacteria to get a tiny foothold.
If I’m right, then a NO₃-PO₄ ratio of 10:1 gave no Cyanobacteria, but when that ratio was 5:1 it could just start to survive. I’m now at a ratio of 7.5:1 and I’m still cyano-free.
It also looks like green dust algae might also be exacerbated by a low NO₃-PO₄ ratio.
Note that I’m not suggesting that plants care about the ratio. I think from their point of view all they want is “enough” NO₃ and PO₄ - but some types of ”algae” including Cyanobacteria do seem to care.
I’m going to be sticking with 15 ppm/week of NO₃ and 2.0 ppm/week of PO₄ for the time being. My Rotala wallichii has gone a nice pink colour and everything is growing nicely. I don’t feel like taking the risk of lowering NO₃ further, although it might be an interesting experiment to see if the Cyanobacteria were to come back. If I do decide to try it, I’ll report back.
