Think a little more about those sudden and unexplained algae outbreaks that hobbyists report. They report these outbreaks even though in the instances when they occur testing would seem to indicate that all water quality chemistry  parameters (especially phosphates) are extremely low (P04 < 0.04 ppm) and within recommended ranges, a set of circumstances where micro algae should NOT grow.

    Such low phosphate concentrations, and the fact that other water quality parameters are completely in line with generally recommended safe levels, should not give rise to the appearance of micro algae at all. Yet, hobbyists report that they, nevertheless, happen and often occur suddenly, at the most unexpected time. This is happening to experienced hobbyists and beginners alike. Everybody is, needless to say, stumped when this befalls him or her, even the experts.

    This being the case, why do these micro algae suddenly appear even though tank conditions and water quality do not account for such growth at all? It has been postulated and even proven that the long-term use of Kalkwasser precipitates phosphates out of the water (which Kalkwasser really does) and that these phosphate based compounds settle on and in the live rock  in the aquarium, as well as in and on the substrate that may be present in the aquarium.

    Since these compounds are not soluble at the high pH levels, they are really not a cause for concern, because in their insoluble state, they are not available as nutrients for uptake by micro algae and the growth of the same. Over time they simply accumulate more and more in the tank as insoluble matter; which can, by the way, be siphoned out from time to time (not weekly, not monthly, but perhaps every six months or so). Insoluble matter is of no concern, since it cannot affect tank conditions. In its insoluble state, it is also inert and does not react in any way with the water in the aquarium. In this state, it certainly cannot provide nutrients for micro algae to grow profusely in some reported cases.

    Apparently, situations can, and may, however, occur whereby certain areas of the tank actually exhibit a different pH level than the one measured in the main body of water in the aquarium. In this instance, "different" means lower, and sometimes quite a bit lower, than the aquarium's pH level.

    Where would this occur and why? Can this, in fact, occur at all? The answer is yes in all cases demonstrated by empirical evidence that such micro algae outbreaks do occur (the reports by the hobbyists). They may not be the norm, but they happen frequently enough to be a cause of concern. Many hobbyists who report that such sudden growths have happened to them even though they have adhered to superior tank maintenance and husbandry techniques and practices. In view of these findings, we have to accept this as a fact. Fortunately the explanation is a lot simpler than one actually thinks. The remedies are not difficult to implement, either, as we shall see.

      The conditions that could lead to those outbreaks are really quite simple, when one thinks about it for a while and make a lot of sense from a chemistry and nutrient availability standpoint.

     Phosphates that are bound in an insoluble form are, therefore, not available to algae. But can and will go into solution because of pH fluctuations in certain areas of the aquarium, areas where much lower pH exists or develops than the actual pH of the aquarium. This is due to nothing more than basic chemistry principles, but principles we may have overlooked up to now. You may recall, though, that in books it is often stressed that rocks should be regularly cleaned off and that any precipitates should be removed. The recommendations had to do mostly with removing organic material that may otherwise decompose. It turns out that these regular cleanings had an additional benefit, removing insoluble, phosphate based compounds, thus preventing them from fitting into the scenario described here.

     When pH drops in those areas of the aquarium, these insoluble compounds break up with the result that orthophosphate goes back into solution, giving rise to the sudden appearance of micro algae growth, a growth that was totally unexpected and unpredictable given the tank's water quality conditions.

      As indicated, this falling pH syndrome occurs mainly in two types of areas in the aquarium: in the substrate and in the crevices in the live rock. Pollutants may build up in those areas and create conditions favorable for the pH to drop quite a bit (even though the drop is very local and does not spread to the rest of the aquarium). These localized drops in the pH level in the areas mentioned dissolve the phosphate compounds that are present there, phosphate compounds that were previously inert and now no longer are. This process puts orthophosphate back into the water. If this scenario occurs in enough areas of the aquarium, enough phosphate may, and will, re-dissolve for micro algae to suddenly start reappearing in the tank. Remember, it does not take a lot of phosphate for micro algae to grow. Levels as low as 0.05 ppm can bring about outbreaks that then need to be eradicated.

     Strong current in the aquarium, cleaning the rock, especially the crevices in those rocks, general maintenance and husbandry all work toward eliminating this situation from occurring. The continuous use of phosphate- and silicate removing compounds will certainly help as well. The silicate removing compounds prevent the reappearance of diatoms (brown algae growth). Changing compounds frequently enough is the key to getting them to adsorb any phosphates and silicates that may developing in this manner in your tank.

     The products mentioned should be changed at the slightest increase of phosphates in the tank or at the slightest appearance of tiny hairy or stringy filaments on the glass or acrylic panes of the aquarium. Most hobbyists, unfortunately, forget that these compounds do not last forever. The load in the tank, the type of filtration used, the amount fed, and the maintenance schedule you adhere to all contribute to either lengthening or shortening the life of these products. Test regularly and  inspect your tank often. If you see any alga growth, even if it is very minimal, change the compounds immediately.

    Cleaning the rock in the tank is not complicated and can be done while it's in the tank or by removing it and doing so externally.

Cleaning the live rock while it is in the aquarium.

    Using a length of flexible hose of approximately half inch inside diameter, create a siphon that leads to a bucket or a vat placed on the floor. Move with the hose from rock to rock and siphon off whatever detritus will come loose. Be careful not to suck any animals into the hose. To greatly reduce the amount of water you are removing, pinch the hose closed when you move from one area to another. Siphon out whatever lies on the bottom as well. Try to siphon between, behind and underneath the rocks also. This is not easy but, with a little practice, you will be amazed at how much dirt and detritus you actually remove from the tank. Watch the bucket or vat and empty when necessary. Keep siphoning until you see no more dirt or detritus.

Since a good amount of water will be missing from the tank you will need to top off the aquarium. The water you use to do so should, ideally, have been prepared the day before so it time to age and all chemical reactions that need to take place have done so. Newly prepared saltwater added immediately to a tank creates stress due to the chemical reactions between the dry salt and components and the water. It takes from 18 to 24 hours for a batch of saltwater to stabilize itself. Additionally, if chlorine is present this process will eliminate it through simple aeration of the new saltwater mixture. There is no need to add dechlorinators to the water.

Cleaning the rock outside the aquarium.

- Have two buckets or vats filled with saltwater ready.
- Have towels around
- Place the buckets on a water resistant material [e.g. Plastic sheet]
- Take one rock at a time. Place it in the first bucket and swish it around holding it firmly with your hand. As you do this the dirt that is present on the rock will come loose and mix with the - Now rinse the rock in the second bucket and then place it in the tank or in a holding vat.
 - Do this with all the live rock in your tank [ or as much as you can ].
 - Before replacing the rock in the tank itself, give it a good cleaning using a powerheads output to rinse off the rocks and all detritus. This will greatly improve the water quality.

Conclusion:

    Micro algae can reappear when phosphate compounds re dissolve in the water, especially if this occurs in several areas at the same time. We have seen that a low PH in those areas is the cause.

To avoid this from happening you should ensure good strong current inside the tank, and clean the aquarium and the live rock regularly. Additionally, about every 6 months you should really give the tank a complete clean out to remove whatever precipitated phosphate compound may be present.