The longevity of GFO differs from tank to tank. It can be determined by your fish stock, the amount of food they are fed, your dedication to cleaning out your filter socks, or even how long you have had higher levels of phosphate in your tank. Assuming phosphate levels have been correctly maintained, you can expect GFO to last from 4 – 6 weeks before it needs to be replenished.
If you have not used phosphate absorber before, or you have let your phosphate get out of control, then the timeline is completely different. When you use GFO for the first time, or to bring down higher levels, it must be understood that until the GFO was introduced, the phosphate had to go somewhere. That somewhere is normally your live rock. You may have heard the phrase ‘phosphate bound to live rock.’
To regain control of the phosphate levels in your system, you need to withdraw the rock-bound phosphate too. You may notice when you first use GFO that the phosphate level does not drop. This is because the GFO has already reached maximum absorption. You will need to replenish your GFO every 3 days for a minimum of 3 to 4 times to absorb the phosphate from your tank. Once you notice a drop in the levels when testing, you know you are close to control. Aim to keep phosphate between 0.03ppm and 0.05ppm.