I don't have a PO4 test kit, so I have no idea what's going on in our 120g. However, there are various macroalgae growing well in the tank, which, combined with loads of light, I am assuming is consuming lots of PO4 and nitrate.
I also do 15% water changes, BB siphoning, and mechanical fitration cleaning weekly. But I doubt that changing such a small amount of water does much to reduce any nutrient could be in my tank. Say, for instance, I had 10 ppm nitrate in there. A 15% water change will only reduce the nitrate to 8.5 ppm. And by the next week, more nitrate will have been added to the water, so I'd probably still have 10 ppm.
All I've got to say is using non-invasive macroalgae to soak up nutirents is the most natural and cheapest way to go.
Am using RO/DI water, btw.
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