RO/DI won't remove all Silica!!

[Bourha]

I'm hoping someone can help me with this problem. I have an RO/DI system that I use for my newly established reef tank and I cannot get rid of the Silica in the tap water. I'm using the Red Sea Silicate test kit and the tap water is showing 2+ppm Silica (TDS 157). After the water goes through the RO membrane it reads 1ppm (TDS 7), but it still reads 1ppm silica (TDS 0) after the DI as well. I have since added Spectrapure's SilicaBuster cartridge to my setup. Now I have two DI's at the end of my filtration. The water still shows 1ppm (TDS 0) of Silica even with the 2 DI filters!!! I talked with Spectrapure and they said that the Red Sea test kit is not accurate, and that I should look at buying the Hach Silica Test kit ($80). Well, the test works fine on my aquarium water (Silica 0ppm) so I don't think the test kit is the problem. I'm achieving the 0ppm in my tank by using a phosguard/silica removal. I want to get rid of the source, but nothing seems to be working. Any advice would be helpful.

Thanks.

[Randy Holmes-Farley]

My expectation is that if you have two functioning DI filters in series and still detect silica, that it is most likely a kit error. While silica can be among the things that penetrate a DI, getting through 2 in a row seems unlikely to me, especially at those levels.

Just because the kit works in salt water doesn't mean that it works in fresh water. :)

Do you have a diatom problem?

FWIW, I dose silica to my aquarium:

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

[Bourha]

No problems with diatoms so far, but the tank has just completed its cycle. I'm just trying to eliminate them from happening. The silica in my tap is 2+ (probably more like 12ppm) so I had to get an RO/DI filter to get it under control, but I was trying to figure out why it wasn't removing all of the silica. I also have about .5ppm of phosphate left in the RO/DI water as well. Maybe my Red Sea Phos test is also giving me a false reading. What is a good test kit for Phosphate & Silica without paying $80 per kit?

[Randy Holmes-Farley]

The TDS after the first DI is 0 ppm? And then a second new one is added? The phosphate should also be zero after both the first, and most certainly the second. Phosphate, unlike silica, is easily removed by a DI.

FWIW, I wouldn't chase silica values unless you have a problem.

[Bourha]

Yes, the TDS after the first DI is zero as well as after the second DI. That's why I can't figure out why I'm reading phos & silica values. Evidently it must be the test kits I have!!

Any recommendations on test kits?

[Randy Holmes-Farley]

Hach kits are good for silica and phosphate. Others probably are as well (Salifert, Seachem, LaMotte), but I've not used them much for these parameters.

[Bourha]

I'm off to get some new test kits.

Thanks for the help.

[Randy Holmes-Farley]

You're welcome.

Good luck!