I started using 500 ml on 6/30/10 and I also ran GFO. I had a bad algae issue. It took changing the GFO every week for about a month to fix the issue. I still have some minor algae, but no where near where it was.
Now, as for the pellets, I'm not sure they've done anything for me. After 5 1/2 months the pellets are still the same size and the same quantity. My PO4 & NO3 appear to read 0 - 0.5 NO3 & 0.00 - 0.08 PO3. My chaeto has even started to stop growing, not sure why. My friend added his about a month before me and he started with a liter and had to add another 500 ml a few weeks ago. His old pellets are about 1/2 the size of the new ones, so his show signs of working.