[quote name='Bob' date='Nov 30 2007, 04:57 PM' post='24937']
Aren't diatoms a type of green algae?/quote]
[Disclaimer: I'm not a scientist and I'm typing this from memory without benifit of references so please step in if you edakated folks see me go astry

]
No, diatoms are single cell organisms that have a "shell" made from silica [hence diatomaceous "earth" is superfine sand- like silica material made from dead diatoms]. There has to be a sufficient amount of disolved silica in the water for them to form. A sandy substrate is usually enough to provide the disolved silicates needed.
Unlike algae, diatoms don't get out of control and blanket everything with thick layers of slime. The coverage on glass and decorations and substrate amounts to a thin brown coating which gives the tank a natural "lived in" look, at least to my eye. Light colored sand substrate does get a dingy look after awhile but if you stir it up, it goes away. Usually all you have to do is scrape the front glass once and awhile and ignore it in the rest of the tank and it can grow for years without really causing any aesthetic concerns [unless you're a clean freak who prefers a sterile looking artificial habitat for your critters

].
I'm NOT a clean freak and am lazy when it come to nonessential tank maintenence so I actually prefer diatoms over algae. That is one reason I go with sand or sand/gravel mix in all my tanks. I believe [but have no supporting data] that a healthy growth of diatoms in a moderately lit tank locks up enough of the dissolved nutrients to keep yuckie algae from getting established.
One interesting observation I have made is that diatoms form quickest and grow thickest in strong current. I have learned to not have powerheads or filters blowing directly onto the front glass. This is true in both fresh and salt water. I would assume different types live in fresh or salt, but if so, they share that trait.