I recently posted a thread about my brown algae(Diatoms)..My aquarium has been up for 4yrs now....40g breeder..no plants...I have never tested my tap water for phosphate and silicate ..I been having issues with diatoms it started about 3 weeks after it was cycled ...so today I went to a few lfs in my area(Colorado Springs)...I told them about my issue with Diatoms...all of them told me our water supply has phosphate and silicate in it..i never knew about checking for it..till now...they told me to use seachem phosguard in my HOB filter...so they gave me a right size premade bag of it for my filter...I understand the directions..my question is how long does it take affect and how long is good for and how often do I need to replace it