Hello,

I have a 240 gallon reef tank that has recently developed a problem with diatoms. I did some research and found that diatoms feed off of silicates, once the silicates are gone the diatoms are gone. So I went out and got a TDS meter and started testing water. My city water is pretty bad having a TDS reading of right around 1000ppm. I then tested my RO/DI water and noticed it was right around 200ppm, which I think is still too high. I recently bought a change of filters for my Kent Marine Hi-S 60gpd and tested that water. The TDS reading did go down but not as low as I expected, it dropped to about 30ppm. I have read that other people change their RO/DI filters when the TDS reading gets above 0ppm. I also checked on the Kent Marine website and found out that my RO/DI unit is optimized for TDS lower than 200ppm. So my question is can I simply put some sort of pre-filter in front of my RO/DI or do I need to get a new RO/DI system? What are my options for fixing this problem? If I need a pre-filter what kind of filter do you recommend and if I need a new RO/DI what brand/model do you recommend?
Thanks for the help!

you could add another DI canister just after the 1st one. that should handle the left over TDS that are getting through. it would be a low cost fix to your problem too.

how many prefilters are there on your unit? The fact that changing them dropped your tds from 200 to 30 is a good thing. A perfect membrane with 98% rejection (Dow 75gpd) would get you a minimum of 20 TDS before the DI stage. After Di with such a unit should get you near "0" until the resin gets exhausted.

TDS meters need to be calibrated against a known 0 tds sample. It could be off even if it's new.

Dick

Thanks for all the comments. I will check the TDS meter, I didn't think of calibrating it since its new. My current RO/DI unit has a total of 4 filters first of which is a 1 micron filter followed by the membrane filter, 0.5 micron carbon filter and a DI resin filter. Do silicates get removed in only the DI filter or are they removed in the prefilters as well?

I think that the membrane and the DI remove silicate.