Almost certainly the problem in that tank is CO2 distribution. It all looks very clogged in the photos although that nay be just an optical illusion from the angle of the shot. Hoppy, I think that second shot shows Ammania Senegalensis, or has it been re-classified as a Ludwigia? In any case I've always thought it to be an agonizingly slow grower but it seems to like lots of nutrients and lots of CO2. My specimens sat in the tank looking meager for months until I trimmed and reworked the tank.
I've attached a couple of quick images to show the difference carbon makes, and to show what this plant is supposed to look like. The first shows an upper area of the plant after reworking CO2, flow rate, doubling dosing and adding Excel. That top section is about 4 inches and that is just in the last 2 weeks. The second image is in an area that has some of the most subdued lighting in the tank and is a bud from the original stem. You can see that even in this area what a difference it makes as new growth is poking out from the dark, and from behind a curtain of D. diandra. To me that's even more impressive than the first image because it proves that the so-called "high light" plants are actually "high carbon" plants. A green/yellow drop checker reading means nothing if nutrient/CO2 distribution and circulation is poor.
This revelation came about without the assistance of useless NO3/PO4 test kits, which tell you absolutely nothing and are an utter waste of time and money. It seems to me that too many people believe that they will somehow find salvation in testing. This is a blind path which leads nowhere. The purpose of a technology is to serve us, but things go awry when we wind up serving the technology. This is what happens with test kits. We want to believe them so we wind up doing everything we can to make sure we can believe them, calibrating them, multiple testing - until we get tunnel vision
and focus more on the test kit readings instead of understanding what we are seeing right in front of our noses. As a result we lose the fundamental truth of plant husbandry and we start to worship at the altar of the test kit. The truth is that if plant is in poor condition it's because it is not being fed or it has no access to being fed, regardless of what color shows up in that test vial. Fix either the feeding or the access and the growth will improve. Individual plants may do well because
they have access.
Based on this experience, I would suggest that you give the tank a serious trimming and substantially increase the circulation either by getting a stronger filter or by adding a pump to supplement flow. Use the money you save by not buying a nitrate test kit and buy some Excel instead. Follow the EI dosing schemes and never reduce the dosing, especially as the plant mass increases. Increase you bubble rate to drive the drop checker far into the yellow if necessary and you'll be able to turn the gas off much earlier, say two or three hours earlier. This will reduce fish stress.
This revelation hit me over the head when I tried to verify my canister filter's published flow rate. The filter is rated at 1700 liters per hour. That must be with no media, no head pressure and certainly with no restrictions such as CO2 diffuser. With everything hooked up, and with the pump head 31 cm below the waterline, I ran the filter and put the outlet tube inside an empty 2 liter bottle at waterline level. It took about 9 seconds to fill the bottle. This means that the filters turnover is about (2 / 9 L/sec) * (3600 sec/Hr) or 800 liters per hour. This is less than half the rated turnover. If the target turnover rate in a planted tank is 3-5 tank volume per hour then we are in serious trouble if we depend on manufacturer's claims regarding the filter capacity. We are in even more dire straits as the tank grows in because this means more restricted flow around each plant.
I also decided to ditch my "lily pipe" outflow since they merely dump the flow into the center of the tank, creating voids at the sides and in corners. I've found that spraybars mounted at the back sets up a better distribution as the flow heads towards the front glass and is then deflected down. This brings CO2 towards the carpet plants in front. The flow then continues towards the back glass and thus comes into contact with the midground and background plants. After everything is adjusted, most if not all plants should "sway in the breeze". I think that this is an indication that each plant has a good chance for proper gas and nutrient exchange, and that you have reduced the possibility of localized NH4 concentration buildup.
Cheers,
