Your Po4 has gone up because you have removed the GHA which was consuming the Po4 your No3 is fine.
Your must be or have been feeding to much and your rocks have absorbed Po4 to a point that GHA has started to grow and kept your Po4 at .12, now you have removed the GHA the Po4 it's not being consumed and has spiked to .3.
If you did a 50% water change it would drop your Po4 to .15 but then it would go back up as more Po4 leaches out of your rock.
Ways to try and fix this:-
Carry on as you are keep removing the GHA and increase your lighting period on the refugium some run their light 24/7.
Setup your GFO reactor, GFO can be messy and expensive.
You could keep doing 50% water changes every day or two until your Po4 stop rising and keep removing the GHA(could take weeks even if doing 50% a day).
The only thing I wouldn't do is use Lanthanum Chloride.
You need to sort out why your Po4 is high though.
Good luck.
Your must be or have been feeding to much and your rocks have absorbed Po4 to a point that GHA has started to grow and kept your Po4 at .12, now you have removed the GHA the Po4 it's not being consumed and has spiked to .3.
If you did a 50% water change it would drop your Po4 to .15 but then it would go back up as more Po4 leaches out of your rock.
Ways to try and fix this:-
Carry on as you are keep removing the GHA and increase your lighting period on the refugium some run their light 24/7.
Setup your GFO reactor, GFO can be messy and expensive.
You could keep doing 50% water changes every day or two until your Po4 stop rising and keep removing the GHA(could take weeks even if doing 50% a day).
The only thing I wouldn't do is use Lanthanum Chloride.
You need to sort out why your Po4 is high though.
Good luck.