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>Having said that, I think the magical 'yet' has arrived.  I'm
>with diatoms on the front glass of my tank.  It hasn't affected my
>but in the past few weeks, I've had to scrape the front glass too
>The FAQ says this is caused by a lack of light or an excess of
>and recommends increasing your wattage.  I've recently stepped up my
>lighting, to 200 watts (on a 75 gallon), so in this case, I doubt it
>help.  Silicates?  What might I have?  I have no rocks in the tank,
>wood, plants and a flourite and sand mix.  It's a new kind of sand,
>that be the cause of excess silicates?

I'm going to have to go against the FAQ here.  Diatoms have high light
requirements, so increasing your lighting will not eliminate them.  I
have always seen diatoms growing in the brightest part of an aquarium,
never the darkest.  Back when my tank was dominated with large,
shade-producing plants, I could go months without having to scrape
diatoms from the front glass because of the shading.

Diatom "skeletons", called frustules, are composed of hydrated amorphous
silica, SiO2.nH2O.  Silicic acid, H4SiO4, is the form of silicon taken
up from the water, and is the major form of silica in solution.  The
weathering of silicate-containing rocks and minerals produces silicic
acid.  Silicon is one of the most abundant elements on Earth, so it's
rather difficult to avoid.

Your tap water is most likely the source for your silica, as quartz sand
is extremely insoluble (so is your glass aquarium).  I suppose you could
invest in a R/O water system, or a silica-absorbing resin, but I believe
the most cost-effective method is to regularly scrape the diatoms from
the glass, and let your filter take them out.  Diatoms are very good at
taking up silica from the water, so if you regularly remove diatoms, you
are also removing silica.  Note that diatom skeletons are one of the
most soluble forms of silica, so removal is important.  Diatom skeletons
are a very important component in the cycling of silica in natural
systems.  Diatomaceous filter media can also introduce a significant
amount of silica.

Keeping your aquarium phosphate-limited helps, also.