Growing Macro Algae in the Aquarium and the Problem of Phosphorous

Nobody in MLSSA to the best of my knowledge has grown macro algae in an aquarium virtually from scratch. What usually happens is that large amounts are collected, placed in the aquarium where they slowly decay, possibly after a period of some growth, with the tips going white. This does not have to be the case. I believe I am on the verge of a breakthrough in this area and currently have Caulerpa, that had completely died in the aquarium, re-growing from stalks with a rich deep green colour. The growth I have is now about 75mm long and about 20 to 30 stalks have started growing. The tank water is crystal clear to an extent never seen before.

Initial Research

Using the Internet, I have collected papers on “tropical reef” aquaria. From these papers I have attempted to work out the key issues applying to temperate marine aquariums and to apply them to my aquarium. One of the major problems preventing the growth of macro algae is the growth of filamentous or “hair” algae which smothers macro alga. It appears that it was not until around 1986 that the major cause of this was discovered – high phosphorous concentrations in the aquarium. Now phosphorous is an essential element for the growth of all algae, but higher concentrations favour the growth of filamentous algae. Phosphorous concentrations should not exceed .05ml/L (loosely ppm, parts per million). My tank measured 2.5 mg/L, 50 times more concentrated! The same applies to nitrates. Nitrates are relatively easily removed from aquaria but phosphorous is not so easy. Even so, my bet is that most aquariums of members harbour high concentrations of both nitrate and phosphate. Nitrate concentrations should not exceed around 3mg/L. Jean Cannon notes that the optimum ratio of P:N for macro algal growth is around 17:1. One method of ridding tropical aquaria of phosphate is to buy expensive, non-rechargeable chemicals such as Seachem’s PhosGuard from the aquarium shop and running water through them. This could cost several hundred dollars a year for large aquariums and is out of the question for most people. I was determined to find a simple, cheap method for such removal. Having read many articles I was none the wiser. One substance often mentioned in tropical aquaria literature is Kalkwasser or Lime water, (CaOH). This, however, was mentioned as a method if increasing calcium in the water which is good for calcium rich corals, and is also good for maintaining Ph. At first I ignored this chemical. I searched high and low and eventually found that phosphorous is now being removed from sewage effluent by a chemical method. Ferrous Sulphate or Aluminium Sulphate is added to the water at a rate of 0.4 mol Fe/ mol P. Not knowing the effect of Aluminium on fish I decided to try this method in the aquarium using Ferrous Sulphate. I purchased a 1Kg bag of Sulphate of Iron from Bunnings Garden Centre. It is a fertilizer and iron soil en-richer. It is only 30% pure but I thought it was worth a try. I mixed up a solution of 2mg/L and let it settle. I was left with a clear solute of FeSO4. I calculated the correct amount of solute to add to remove all the measured phosphorous in my aquarium. It was 160ml. It was do or die. I put it in the aquarium! To my excitement next morning, my aquarium was crystal, crystal clear and the phosphate level had dropped by half. The following morning it had halved again. My sea urchin had come out and my remaining starfish got its colour back. I thought I had made a breakthrough! However, such was not to be the case. My pH had dropped from 8.3 to 8.0. Ferrous sulphate was having an acidifying effect on my aquarium. I tried to correct it by adding Lime Water. Disaster next morning. The phosphate was up as high as it was before and the pH had not moved. The acidic sulphate was reacting with the alkaline CaOH. I was stymied.

The Discovery

I went back to my books. In sewage effluent phosphate removal can be achieved by the presence of Al++, Fe++ or Ca++. Searching under calcium I found one author that says, “Limewater precipitates PO4 out of solution.” (http://www.athiel.com/lib/jaubert.txt) Bingo! This was confirmed in another article where limewater precipitates phosphate over a period of five days. (Notes from the lab: Kalkwasser and Phosphate, http://www.nauticom.net/users/michaelh/notes.html). I have yet to confirm this experimentally, but initial trials over two days appear to be confirming this. This being so, then we have solved the problem with phosphate without acidifying the tank. In addition to this I have found further information that some aquarists “resort” to the use of ferric chloride, a very dark powder or crystals, to lower phosphate. This reaction takes place in a few hours and a sludge like precipitate, probably insoluble Ferric Phosphate, builds up on the bottom. Unlike Ferric Sulphate, Ferric Chloride has an alkaline reaction in the tank. I think limewater is safer.

How to make and Use Kalkwasser

Kalkwasser, limewater, is made by mixing builders hydrated lime with distilled or rain water. The solubility of calcium hydroxide is 1.5g/L at room temperature. Handle the solution carefully as it is caustic with a pH of 12! Mix in the correct proportions and let it settle in a sealed container brim full for five days. Then decant the clear liquid, which will already have the original phosphate in the water precipitated out over the five day period. Rinse out the original container and prepare a new batch. Use this water to replace all evaporated water. Do not add it all at once. Let it drip in from a decanter as a permanent thing. Check pH regularly to ensure that the additive rate is correct. Be sure that the settling container is sealed as the carbon dioxide in the air will combine with the calcium ions in solution and precipitate out calcium carbonate, which is shown by the liquid becoming milky. Buy a phosphate test kit before you start to confirm your results.

Additional Factors

Macro algae requires trace amounts of iron in solution for good growth. Do not use ferric sulphate or chloride to achieve this as the iron is precipitated out as an insoluble phosphate. Use iron chelate, which remains in solution. The correct concentration is 1-2 ppm. Exceeding this dose may not effect fish but will effect some invertebrates. A general marine algae fertiliser will also not go astray. Marine Green by Aquasonic, made in Australia, is reasonably priced and is available from the Plympton Aquarium shop on Anzac highway. Finally, watch your nitrates- but more of this later! Author: Peter Gilbert.