That’s good news
If you want to raise nitrate just maybe feed more or add some more fish, if you have zero nitrate just watch out for cyno bacteria forming which may be the red stuff you see as it can thrive in ultra low nutrient environments.
I would read up on mixing the GFO and carbon as I seem to recall it’s not advisable due to the different particle size and bashing together but it may depend on the brand etc.
I have the carbon bag (around 200grams) in the holder just before the return chamber, so you don’t need it in the reactor anyway, that’s how it was designed by Red Sea