Diatoms and Silicates

[CapitalO]

I have a 29g LPS reef and I can't seem to beat back my diatom problem.. The tank has been established for almost a year now and I still see diatom buildup on the glass, becoming noticible if I let it go for a few days without cleaning. There is far less growth on the LR, mainly on the glass or high flow areas, such as pump outlets and the overflow. the growth starts to turn hard and encrust if left alone. I have no problems with any other microalgaes, my nutrients are very low and parameters are well in check (ammonia, nitrite, nitrate, and phosphate are undetectable, reading 0 on my tests. My Ca stays around 380-400, Carbonate alkalinity is about 2.5 meq/L, pH 8.3, SG 1.0245) I dose seachems reef complete, CaOH2, kents trace elements and I use seachems reef buffer for kH and pH. I have a 20g sump with a CSS 65, photo fuge (with chaeto and a DSB). Since my water doesnt have a detectable level PO4, this leads me to believe that the diatoms are feeding off silicates which i have not tested, i have heard these tests are often misleading and i havent wasted any money on a silicate test. I also stopped using my RO water for about 2 months and started using distilled water for top off and not doing water changes. This did not affect my diatom blooms, so i highly doubt the source is my makeup water.
I also use Phosguard (which absorbs PO4 and silicates) and there is a noticible change in the rate of diatom growth which eventually increases again when the phosguard is exhausted.
I have about 2 inches of sand in my main tank, which is a mix of coarse and fine grain aragonite live sand and very fine (sugar sized) white silica sand. I have heard that people have reported diatom blooms in tanks with silica sand (I know silicon dioxide is insoluble in water and many beaches are mainly composed of silica sand as well as the glass in our aquariums).. Im curious about how silicate ions found their way into my tank.. so any feedback would be great

[Randy Holmes-Farley]

Diatoms need phosphorus as do any organism, but they certainly can be controlled by reducing silicate too.

If this has been an ongoing problem, I'd get a silicate kit and test the tank water, water change water, and the distilled water. Maybe you can track down the source.

Silica sand can release silicate, as I show in the article below:

Silica in a Reef Tank
http://advancedaquarist.com/issues/jan2003/feature.htm

[CapitalO]

Thanks, that article is exactly what I was looking for, great stuff! So the SiO2 in the silica sand slowly reacts with H2O to form Si(OH)4 which the diatoms use to form their skeletons. I also have an abundance of sponges in my system as well which would indicate soluble silica I suppose. Do you think that after the system ages further the rate at which the silicon dioxide dissolves will decrease? and does the fact that I have diatoms indicate that my level of phosphates are too high rather than the silica?

[Randy Holmes-Farley]

I wouldn't yet be sure that the silica sand is the issue. tap water is often a bigger source.

I do expect that silicate released from silica sand will slow down as readily dissolved silicates are depleted, leaving much less soluble quartz.

Problem algae need all of phosphorus,nitrogen,and in the case of diatoms,silicate. Cutting off the supply of any can work.

[CapitalO]

Could you recommend a good silicate test that wont break the bank and have some accuracy? Also, thanks for the guidance you have been very helpful!

[Randy Holmes-Farley]

The only one that I've used is the Hach silica test mentioned in the article.

Anyone else find a less expensive one that they like?