Chapter IX-Cyanobacteria control by reducing dissolved organic compounds

From week 62 on I just did 37% water changes/cyano siphoning/gravel siphoning every two or three days and nothing else. I no longer had any hope that the cyano would be affected, it was just so I could bare to look at the tank (and to reduce the smell). I remember a brief period when I did water changes but didn't remove any cyano with the water (except to scrape it off the tank walls). I think the idea was that the cyano population would grow so large that it would use up whatever it was feeding on and 'crash'. I lasted about 10 days but the tank got so disgusting I couldn't stand it. I also worried that if the cyano did crash and start rotting, it would use up oxygen in the water and harm the fish. I also carried on reading.

I can't remember where I first read about dissolved organic compounds (DOCs), I think it was on one of the reef forums. Reef tanks, and salt water tanks in general, also get cyano problems. Usually the cyano is red so it's often called red slime algae. Once I started searching for DOCs and cyano I discovered a whole new world where  it was well known that DOCs had to be removed or algae and cyano problems would occur. That's what protein skimmers were for, and from what I read they were pretty much universally used in the reef world. Another technique recommended for reducing DOCs was the use of activated carbon.

So what are DOCs? Basically they are a pool of organic matter at various stages of decomposition. Everything that had ever died in my fish tank (bacteria, fungi, algae, plants) and all the fs would have contributed to them as they decayed. Some DOCs are short lived and are broken down to compounds that can be used by plants (e.g. nitrates) but apparantly some degrade slowly and can persist (e.g. tannins) 

It made perfect sense to me, the food source for my cyano was DOCs. Maybe, I thought, cyano could use some of the slowly degrading complex DOCs as a food source but algae couldn't, and that was why cyano always seemed to dominate my tank. I guessed the DOCs were mainly coming from the dirty gravel. I had done a pretty good job of cleaning it but it had apparently not been enough. I also started reading criticisms of under gravel filters (UGFs). The criticism was that UGFs make it harder to keep a tank clean. Because any waste is drawn down into the gravel it is harder to siphon off and, even if you keep the gravel clean, you can get a build up of decaying matter under the filter plate. In week 73 I installed an internal canister filter and left it for a month. The filter had two carbon/wool filter pads, two sponge pads and a compartment for 'biomax' pellets (pellets that encourage the growth of nitrifying bacteria).

Week 74. Still from a video one day after a water change. The canister filter is
installed and the gravel looks clean-ish. I notice there's hardly any Ellodea left. 

Then in week 77 I turned off the UGF. I removed the riser tube, blocked up the hole in the filter plate and carried on with the water changes as before. I hoped that because the tank water was not being drawn through the gravel bed, that less DOCs would build up in the water. Or that maybe the different conditions in the gravel would result in a change in tank chemistry, perhaps due to different types of bacteria living there. I also hoped that the carbon in the filter and mechanical filtration would reduce DOCs to a point where the cyano at least slowed down in it's growth. It didn't. I thought OK, if the gravel is dirty enough then turning off a UGF won't do. Maybe you reach a point where there's so much organic matter in the gravel that you always get sufficient DOCs for a cyano bloom. I decided to start again, but this time to use a fine grained gravel so any fs would just lie on the surface and be easy to siphon off.

In week 80 I siphoned off 20 liters of water and put the canister filter and the fish in the 20 liters. I then removed the remaining plants and treated them with antibiotics before completely breaking the tank down. I removed the gravel and UGF. I should say that the gravel and filter plate were absolutely filthy despite all the siphoning etc. I scrubbed  the tank and fittings (no chemicals) and put in new fine grain black gravel, 34 liters of water then the canister filter and fish with the 20 liters of original tank water. The fish seemed fine after they calmed down. A couple of days later, I washed the plants and put them back in. Weekly water changes as before but I used a new gravel siphon, one of the ones with the wide riser tube that the gravel goes up a few inches while you're siphoning. I found this was the best product I had ever used for gravel cleaning, you could see it siphoning off everything. You may be wondering if cyanobacteria returned to my fish tank and if so how long it took? The answer is it returned in six weeks.