Now that I think about it, if low K was preventing N and P uptake, then the plants could be starving yet test results might still show seemingly adequate N & P levels. I had never dosed much K since I'd thought that enough was present in the KNO3 and KH2PO4.
See my analysis as to why if you dose KH2PO4 and KNO3, you should never be low on K+.
Sounds more like the issue if you did not dose enough or dose enough CO2.
Sediment may have nothing but correlation to do with it(as is often the case).
I have client's with nice tanks using EC as well as ADA AS.
If you move things around, rescape and do not want to replace the sediment, then EC works, so does Flourite black sand, I prefer that but cleaning it is a PITA. EC, no cleaning, but looks a bit tacky really.
Tack if you still live in Sau, let me know, I can stop by sometime, I use to live in Larkspur for 15 years and my sister still does.