Algae can be a friend to the aquarist, either in the sump or even the display aquarium, in the control of nitrates and phosphates. Caulerpa is the type normally used for this purpose. Encrusting [tag-tec]calcareous algae[/tag-tec] is also a welcome addition, as it appears in lovely colours and enhances the display. Algae can also be a pain, in some cases causing the aquarist to nearly tear his/her hair out.
The very first point when discussing algae control is that the seawater should be of high quality, with very low measurements of nitrate and phosphate. Nitrate should be below 10ppm (parts per million) particularly in a reef system, (a fish only system can go somewhat higher but levels should be minimised) and phosphate not higher than 0.03ppm. It is probably better that the levels are undetectable. Nitrate and phosphate are known nuisance algae nutrients. To assist desirable calcareous algae, calcium and alkalinity should be controlled. In a hard reef aquarium, calcium will be probably added, but other systems can benefit from around 400ppm or higher as well. Alkalinity can be kept at a level (it will fluctuate) of 3.0 to 4.0 meq/l – this is higher than natural seawater but is advantageous.