Why Are My Plants Turning Brown?

Look at the ferns which started out a bright, unblemished green, and the cardinalis which used to be a cheery red on the top of the leaves. The tank and the plants are a month old. Even the new water sprite has started to brown on the top.
The tank started with Flourish comprehensive and Flourish Excel, and now I have switched to dosing PPI-Pro macros and micros. A few days ago I started dosing DIY CO2 using the citric/soda reaction. My drop-checker reads green for much of the day, I stop CO2 in the evening. My light is a Finnex Planted+ 24/7 CC, which is on a 24-hour cycle that includes a photoperiod of maybe 5-7 hours, tough to tell with the intensity and hue fluctuating.
My water chemistry is good for the nitrogen cycle, KH is 7, GH is high at 16. Phosphates untested, but Nitrates 5 to 10. 60% water changes every week. pH is 7.2, but pulls down to as low as 6.9 after a few hours of CO2 bubble. Recovers overnight.
I cannot find these particular symptoms of browning and weakening of leaves in the deficiency list.
Am I burning the leaves with too much light, at that photoperiod? Should I come down to just 4 hours? The bottoms of the cardinalis are still rosy red, just the tops are brown.

 

Most likely your phosphate level is high I would say around 10.0 try to use phosphate trap guard in your canister it will drop you level you want it around 2-3 so plants will be happy ..

 

Most likely your phosphate level is high I would say around 10.0 try to use phosphate trap guard in your canister it will drop you level you want it around 2-3 so plants will be happy ..

High phosphates do not cause plants to die back. If his phosphates were high you'd see excessive algae growth as well.

Java ferns are low light plants and do better in a shadier tank. It may be possible they're being blasted by excess light.

All the pinholes and discoloration, however, look more like nutrient deficiencies.

 

I do have some algae on the walls, and a few strands of hair streaming out from the HOB filter, so I will test for that tomorrow.
What about the Telanthera Cardinalis? The top surfaces have browned from bright red down to that dark bronze. I thought they were considered high-light plants.
As for nutrient deficiencies, the ferns and cardinalis were present to be abused over the last month. But my water sprite is browning on the upper leaves too, and it was planted in the tank after I had started PPI-PRO. I'm not arguing against anyone's diagnoses, just trying to account for all the phenomena I'm seeing. I guess the first step is to add the phosphate level to my model.

 

Well, I had a hunch, and got wet, and what looked like brown leaves turned out to be brown algae ON the leaves. Meaning it's been accumulating for a few weeks, before my hair algae and green algae spotting on surfaces even developed. I was able to rub some away, and there's healthy green leaf underneath. At least, that's the case for the ferns. The Telanthera cardinalis does have real bronzing on the top surface of the leaves, and rosy undersides.

So, I read here a bit and it sounds like the answer for the brown algae is to increase the photo period over the current 6 hours? Let the plants go nuts with light and co2 and fertilizers and outcompete the algae?
And...what about the cardinalis? Trimming isn't an option, every leaf is affected. I don't know if a change in environment brings the red back, or if they are doomed to a colorless life. Anyone seen cardinalis recover in this way?

 

And, ZigI Zig , I just tested phosphates and as you predicted I am off the chart, at least 10ppm. I don't have a canister filter so I can't use your solution, but now I have a target. Thanks!

 

And, ZigI Zig , I just tested phosphates and as you predicted I am off the chart, at least 10ppm. I don't have a canister filter so I can't use your solution, but now I have a target. Thanks!

This should work with your filtration