I wouldn't consider those levels high N & P. Mine are far higher. And I know T. Barr has kept heavily stocked tanks at considerably higher levels.
And the question about bio availability and testing results is open to debate. For me, I have to dose regardless of my readings. Over time, I've learned to pay more attention to the plants than the readings. It might be worth a try just to eliminate any deficiencies.
And by the way my tank really is heavily stocked, with 32 fish (Rainbows, Clown Loaches, Roseline Sharks) in a 120G. Like I said, I can get well past your readings without dosing a thing. And on a side note, I've always felt that fish are very adaptable, and the effects of pH on fish are very much overstated. At least that has been my personal experience from decades of fish keeping.
One other random thought. You say you clean the filter twice a week? Just one filter? Have you ever tested for ammonia? Crazy thought but just wonder if you are cleaning filter TOO much, and if bio field is able to stay stable?
I have 20 fish in a 75, including a 10" Vieja Bifas, 10" Green Terror, 6" Frontosa, half a dozen 3 to 5" African and CA cichlid, and 10 bristle nose plecos. These are bulky fish and big eater, so I have to do massive WC weekly to manage the nitrogen cycle. I thought I could never have cichlid and plants co exist, but they have been well behavior, except for digging and spraying the substrate. This is why I concentrate on tough leaf epiphytes only by glueing them to rock. When plant people like Tom Barr say their tanks are heavily stocked, they typically meant light weight schooling fish, not bulky fish like cichlid. A 6 inch cichlid, for example, is roughly equivalent in mass to 100 neon tetra.
I love and had several clown loaches that had grown to old age and large size, but got wiped out by an ick infection. I love Roseline Sharks and Rainbows and want to introduce them to my planted tank, but they must be big enough to avoid getting eaten.
I run two Penquin 350 HOBs which I changed filter pads about twice a week when the media got clogged up. My CO2 is dispensed by a motor driven Tunze Diffuser that adds more circulation. The combined flow is around 900 gph or about 12 times turn over an hour. Unlike canister filter, HOBs never never slow down in flow rate when the media get clogged, only to bypass onto the spillway. I only test for nitrate periodically, and not NH3 for ages as the tank has been set up and running fine with healthy fish for over a decade. BTW, I read that plants prefer uptaking NH3 to NO3, so biological filtration in a planted tank is redundant.
A pH drop of 0.6 means the ppm of CO2 in the water went up by about a factor of 10 to the 0.6 power, or 4. Since the CO2 level may have dropped to as low as 3 ppm when the CO2 is turned off, depending on how much surface ripple you have, you may have as little as 12 ppm of CO2. It isn't likely that you have 20 ppm, but you could have.
Flourish Comprehensive isn't a good fertilizer for NPK. It is a trace element mix, with just a little nitrogen in it.
According to the Rotabutterfly calculator, my CO2 level could range between 10 to 20 ppm during photo period as matching the pH color chart is hard and half guessing. I don't intend to increase CO2 injection rate further to lower the pH below 7 for fear of killing my hard water fish. Right now, I am injecting one bubble per second, and the plants are growing.
My dosing of Flourish Comp is for Fe Glue and trace only, not for NPK. The only macro I dose is 20 ppm K after each WC, as my testing show high level of N and P from fish waste. I also dose extra Fe DTPA on alternate day. The question I am asking is whether fish waste N and P are equally bio available to plants as fertilizer N and P. I learned the hard way that fish waste or rock mineral Fe are not, which got me into dosing chelated Fe.
