Ok that's not bad , fwiw rocks can only absorb and leach Po4 and not nitrates , nitrates are not bound to anything so if your rocks have been in your system for a long time and your running a P04 binding media then it's only a matter of time before they would stop its not a problem you would have for. Year unless you kept adding phosphates and not exporting them.
While I know the numbers are low the algae is obviously getting enough to get by ,you don't dose iron do you?
I had issues with bryopsis and low nutrients my po4 was always bear zero because of gfo but my nitrates were always above zero , still low but still there none the less.
While large water changes work its time consuming and expensive which is what i use to do , I now dose vinegar and no more bryopsis as well no more nitrates either. I have also not had to run gfo in a fair bit of time as now the bacteria have been able to keep it down as well.
Water changes are still important for other reasons but are a poor way of removing nutrients.....1 step forward...2 steps backwards.
Maybe set up a ats, denitrator or start carbon dosing to help keep down the lower amounts?
The worse part about our kits are ....they suck. Not s much at high numbers as we care less how accurate they are when things are Hugh but on the low side the numbers really aren't all that accurate.
So a little help from our oldest friends goes a long way
With gha it's usually very easy to fix but blackouts , water changes, cutting back on feelings etc are all things that help overnight but not long term. As time goes by things find there way back to where they were so you need a long term plan.
Import vs export