Plants look very healthy & algae looks to be minimal, so really a nice low tech (slow growth 😉) set up.
If you feel you want to tinker, you might
1) add another dose of liquid CO2 at the start of the evening light cycle - calculate the maximum that you want to add to a tank of your size, then split this into the morning & afternoon doses
re if it does have a half-life of ~12 hours, then the amount of available "carbon" will already be significantly lower at the start of the second light cycle,
(I've never seen the research data on this so I've no idea how this time was calculated or what effects light, temperature & water chemistry may play - it seems to be bandied about the net as if it is FACT & not CONJECTURE ...)
- also is the "carbon" most available in the initial compounds added to the tank, or does the "carbon" actually become more available as those initial compounds interact/react with tank ecology ...
2) if you're at an excess of "carbon", consider adding slightly more light, eg, turn on that 2nd T5 for 1hour in the middle of your evening light cycle
3) change your weekly fertilizer dose into a daily or alternate- day dosing scheme
You can go back to the Tropica site & read what they recommend on their fertilizer system ... many of the videos seem to use daily or alternate-day dosing with the fertilizers rather than adding a single large weekly (water change day) dose.
When you trim the stem plants, are you seeing good growth from the trimmed base & the replanted top?
When doing these changes/any changes, be very conservative & wait couple of weeks for response ie don't actually increase your total amount of weekly fertilizer until you see a need (eg, pale leaves on new growth), if you add that extra T5 light for 1 hour & see not much happening after a few days, don't add another 30 or 60 min to that light effect, wait the couple of weeks, make sure that the minimal response you're seeing is not growth limitation due to insufficient "carbon" or other nutrients (usually it's CO2 that limits plant growth).
I think you've got a good creature population re CO2 production so I wouldn't add much in that area (if you only had 10 fish, I'd suggest increasing fish #'s as a potential CO2 source)
I find it notable that your fish were gasping when you're not adding CO2 gas to the tank nor is the tank "heavily" planted (there is loads of swimming room - I very much like this layout for the fish), this seems to be a function of the lily pipe system (I use those ugly green spray bars 😉 as I already break enough wine glasses 😳)
