GFO:
Think about it like using duct tape to clean the lint off your pants. The lint sticks to the tape. Once every inch of the tape has lint sticking to it, no more lint sticks. GFO/Carbon works in a very similar way. Carbon has organics and particles stick to it, GFO picks up phosphates.
Why be careful with GFO? Any change in a reef tank that is fast, good or bad, can be devastating to corals. Why would one run GFO only a few hours a day? Because one figures that if his phosphates are high, running it for a few hours a day will only lower it so much and it will not be a shock to the system.
If you use 'less than the recommended amount' but keep the pump on 24/7, well, all you have done is run water through it until it cannot absorb any more phosphates. You haven't really bought yourself anything here as far as the speed of the phosphate decrease, you have just limited how far it will go down. If you put enough in the reactor to absorb every last phosphate in the tank running the pump 24/7 for the next 3 days and then the GFO will be used up, great. However, if you don't replace the GFO for 4 weeks, well then that is 25 days that the phosphates have to build up before you replace the GFO. Then, you put in the full amount and it starts all over, drops it to zero over a few days and then keeps it there slightly longer than the last time. You essentially put your tank on a yo yo where the route down to zero phosphates is fast and the route back up to higher phosphates is slow.
What to do? My advice is that when you start GFO, use the recommended amount, but only run it for a few hours a day, that way, your phosphates don't lower too fast. After a week, maybe you can change to 24/7. Some people complain about 'clogging' of the reactor, to which I say, switch to using half the recommended amount, but change it twice as often. Personally, I run a little more than half the recommended amount, I run it 4 hours in the morning and 4 hours in the evening and I replace it every 2 weeks religiously now.
Starting with less than the recommended amount could work if you intend to step it down, but it is harder to keep up with I think. You'd have to do something like 3 Tablespoons over 3 days and that lowers it from .70 to .60 and then is exhausted. Now, I will go with 4 tablespoons over 4 days and that lowers it from .60 to .45 so on and so forth. Then, if you forget to change it out after a few days, they start to go back up. For me, it is easier to put the reactor on a timer and just control how much water is going through the GFO. Up to the person, I guess. You for sure can't use half the recommended amount but still wait 4-6 weeks to change it out, that will absolutely not help ease into it, in my estimation.
