For fighting diatom algae and cyano, common advices are to lower the light or turn it off, more filtration or flow, raise CO2 and a lot of other peculiar, not working, tricks since they don't alter the reason for the algae.
GSA (Green Spot Algae) is another problem, people often give up fighting or don't really have a solution for.
I have made a discovery concerning the above that I wan't to share with you. I've already described it on a Danish forum, but it doesn't have many visitors. Feel free to pass this information to help your fellow aquarists!
To begin with it's not really a discovery since similar solutions are already used in agriculture, for removing algae from lakes and cleaning water for phosphorus at least in Denmark, but I haven't seen it mentioned anywhere for aquarium use, so the "discovery" is more like realizing it goes for tanks too.
I have a 250 litres tank with granite substrate. I had big difficulties with plants producing "air roots" (roots growing upwards from the substrate into the water) and lots of GSA. At the same time I had lots of diatom algae growing on the substrate, plant leaves, tank walls and decorations. Water changes only made the diatoms bloom again. Air roots and GSA are signs of phosphate deficiency while diatoms and cyano bacteria are signs of too much phosphorus compared to nitrogen in the water - a little hard to understand!
For half a year I dosed ferts without any result. I then added a lot of red clay-sticks:
under the plants' roots and also stopped using ferts. Now, half a year later, all diatoms and cyano bacteria are gone and the plants no more have GSA (or any other algae).
I also have a 54 litres tank with substrate taken from the beach containing lots of shells, lime stone and other stuff contaning CaCO3:
In this tank I've never seen any other algae than normal green algae which I find rather natural and decorative. Green algae are not poisonous and willingly eaten by fishes, snails and shrimps:
So my suspicion that the lime made the difference between these two tanks resulted in a lot of experiments and searching for information. Here's the explanation I've come up with. It might not be really scientific, but nevertheless fits with known physics and observations made on the two tanks mentioned above:
In an aquarium with a granite substrate, which is probably the most common substrate used, fish feces and rotting plant material will over time form mulm, which builds up in the substrate.
This mulm is known to have a negatively charged surface.
Potassium, magnesium and nitrogen in the form of ammonium (K+, Mg+ and NH4+ resp.) are all positively charged and hence will be attracted by the substrate and stay there ready for the plants to use via the roots.
Most nitrogen will exist in the form of nitrate (NO3-) though because of nitrification bacteria's breakdown of ammonia, ammonium and nitrite to nitrate. Nitrate doesn't bind to the surface of the substrate since it's negatively charged and therefore instead repelled by it.
Phosphate is also negatively charged (PO4-) and therefore repelled by the substrate, BUT!: this can be changed in several ways and easily.
An interesting discovery about cyano bacteria is they are present in all free water on the Earth's surface. Whenever the socalled Redfield ratio (about 16 nitrogen atoms to 1 phosphorus atom) tilts too much towards phosphorus, the cyano bacteria will bloom until the Redfield ratio is reached. Diatom algae often live in symbiosis with cyano bacteria which besides oxygen produce ammonia which is quite poisonous to living creatures like fishes, snails and shrimps. In fact I killed two large batches of shrimps before I realised the reason, feeding the shrimps with algae whereof most were cyano bacteria. Don't do this at your home!
Granite substrate doesn't contain much to attract the phosphate. Red clay contains about 3.5 % aluminum- and iron oxides which the phosphate will bind to. This must be the reason why all the algae disappeared. A more efficient way of binding phosphorus is using chalk, lime stone or the like containing about 100 % CaCO3. It does have a disadvantage though since it will raise the kH and pH somewhat. The higher pH will also reduce the uptake of free CO2 causing the plants to grow slowly, but this is easily overcome by adding a little CO2 from a yeast reactor.
Red clay also has a disadvantage since it makes my crypts dark colored - it could be the high content of iron causing this, but I'm not sure - yet.
Other metal compunds will bind phosphate, fx iron sulphate and aluminum hydroxide. The advantage of these over lime is they are pH neutral. Since the iron suphate will probably make my crypts dark too, I've started an experiment with using pure granite substrate with aluminum hydroxide added to do the phosphate binding. Actually I also added som talcum powder (appx. 32 % Mg and 63 % SiO2) to prove that a high content of silicates is not enough to make diatoms bloom as it's often said..
This experiment has only run for 4 days so I haven't a conclusion ready for you yet, but so far the snails, shrimps and daphnia I put in are not showing any sign of poisoning and I've not seen any algae so far.
The thing with aluminum is it becomes quite toxic at extreme pH levels, but the aluminum hydroxide has been tested safe for use at pH levels between 6 and 9 before it was used to cleanup Danish lakes. If it was poisonous in aquariums, the aluminum oxide in red clay would also be toxic, and I have had no problems with red clay in this respect. You probably should avoid aluminum in soft water tanks though since the pH could sink below 6 in such tanks!
So so far my suggestion is to use some lime stone powder. It's available for removing moss from your lawn and probably very cheap also where you live! Over here in Denmark it's about 4 dollars for 20 kilos. I crush it in a mortar to be sure to remove any bigger particles. Most particles are microscopic giving it an enormous surface and as such makes it very effective for binding phosphate. I put 1 (one) gram for every 50 litres of tank in a cup of water and stir well. Then it's just to spread it on the surface of your tank.
The water will be milky for about a day while the lime powder binds the phosphate and sinks to the bottom. After the treatment your water will be amazingly clear though and maybe clearer than ever. If you have a pH-meter it's a good idea to measure the pH before the treatment since CaCO3 is a macro nutrient and therefore will be taken up by the plants over time. When the pH then gets back to the original level you just add some more. Also monitor the plants for signs of phosphate deficiency and the water for high phosphate levels.
One more thing I'd like to share is this: I've stopped using filtration in my tanks. A filter will act much like your soil, meaning important nutrients are removed from the water. The plants do a much better job in cleaning your water and are surely prettier to look at! I have found no evidence that flow helps anything though many people recommend it, so the only flow I now use comes from an aeration stone.
Also notice I don't have to use ferts anymore! An average population of fishes, snails and/or shrimps in a well planted tank produce enough nutrients for the plants, of course dependent on the amount of plants, level of lighting and use of CO2. Feces are said to be low on potassium, but since my plants look healthy it's not a problem, maybe because I feed my fishes entirely with living food like daphnia, blackworms and grindal worms. In animals the potassium is mainly found in bone structure, teeth and the skin I believe, so that could be the reason. Flake food is normally made from algae and some kind of dried animals, so it should contain enough potassium, but I don't know for sure.
In a heavily planted tank with no animals of course you'll need to add ferts of some kind.
Hope this helps you, and please spread the word!
Dane William (that's not my real name)
Edit: corrected some typos. If you find errors or frases that could be misunderstood, please let me know.