Generally speaking Algae is often times both a good thing and a bad thing.  However it fits into the category of either desirable or undesirable but not both in my opinion.

    To give a short example, hair algae is like a plague to many reefers, yet it is invaluable as an indicator of conditions in the tank.  Without it I would suppose that some situations could get completely out of hand before even being noticed at all.  In this example it is both a good and a bad thing.  However it is still a undesirable algae.  Following this idea is how I will classify algae in this section.

    Keep in mind that all algae that I am aware of fall into the overall category of a plant, not an animal.  As such their needs more closely resemble that of a houseplant than the family catdog.

    Of the desirable algae there are numerous classifications and one could get almost carried away just logging them all.  So we will stay with the basics.  Those who wish to learn in depth should seek out further information on the web.

    This group of algae come in many colors, shapes, and needs.  Reefers want it in their tank for two main reasons.  The first is that it makes the whole tank look prettier, the other is that it can cover surfaces that nuisance algae would otherwise consume.

    One of the basic things one needs to know about it is that the more bright colored they are the more light they need.  For example, the really bright pinks need lots of light whereas the deep purple colors recede in light that is loved by the pinks.  If your goal is to grow a specific shade of corraline, you need to be aware of it's lighting preferences and then do what you can to provide that.  Also you will want to be aware that any time you upgrade or downgrade your lighting you will notice that a shift in colors of the predominant corraline will occur.  If you increase your lighting to much too fast you could end up whipping out almost all of the corraline you have and it may take some time before a new type establishes itself.

    Another basic is that the thicker the algae grows or the faster it grows naturally the higher their demand on nutrients from the tank water.  Primarily they are eager to gain calcium, hence why they are called calcareous algae.  There are other constituents that they need as well but generally if you keep your Alkalinity and Calcium up where they belong and your lighting consistent you should have no trouble growing abundant corraline on any exposed surfaces.

    This form of algae generally refers to the more plant like of the algae family.  They are often used in refugiums as a means to absorb/adsorb excess nutrients from the tank water.  They are then harvested to completely remove these trapped nutrients from the system.  Keep in mind that some of them can look quite nice in the main display tank as well.  Many fish enjoy actually eating them as a main course or as a supplement to their diet.

    Some of the macros have a habit of doing one of two things that can cause trouble.  The first is taking over the tank.  Sometimes even when you are cropping them often they will sprout up all over the place and you may end up with a dozen or more patches that can be difficult to control.  The second is when they do something called going asexual.  Essentially what this does is release a flood of spores into the tank water that then use up much, if not all, of the available oxygen and cause some real bad things to happen.  

    For the most part any algae that is not either a corraline or a Macro is an undesirable pest.  They crop up for all sorts of reasons and can often be difficult to fight back.  Avoiding any and all undesirable algae is virtually impossible.  Keep your reef going long enough and you will get a bloom of one sort of another.  I have never heard of anyone who has not had at least one battle with a nuisance algae.  I will expand this section as time allows.