In regards to the fish not being affected by high PO4 you are 100% right they will show no real sign of stress although it can lead to a general weakness of teh immune system as the fish are kept in conditions that are not native to them, nitrate in high levels will also cause gill deformaties.
The biggest issue for fish only systems is tank appearance, algal blooms Cyano, to name just two.
Drop me an email at
sales@aqua-digital.com and we can go from there.
Dan - do a heat test on the PO4 then see the results, it could be higher than you really think.