I ran into algae issues first and decided to up the CO2 beyond the shrimp-safe level, since there weren't any left at that point. I'm planning to add more fish so the P level is going to jump anyway. In the meantime I'm boosting the KH2PO4 dose.
I've also cranked the flow from the spray bar so there are ripples and lots of flow, meaning I needed to add even more CO2. The fish are happy with the extra O2 but now I have a waterfall of floating plants.