Bizarre Ludwigia Repens problem!

[leefer]

My tank is 75 gallons, 48x18 inches, about 2 years old. Fully planted with co2 (lime green), two 48" Fluval Plant 3.0 LED, running all colors at about 60-75% intensity. Fertilizer is all-in-one liquid, with liquid iron supplement.

At the beginning, the strongest and fastest growing plant was the Ludwigia Repens. I had to trim the tops every week and replant. It got so dense I would somtimes uproot the entire bunch, and only replant the nice looking tops. The L. Repens was bulletproof!

6 months ago, about the same time as I added Hygrophila Difformis (which quickly grew to be a huge bush and now requires trimming weekly), I started getting problems with the L. Repens. I don't whink it has anything to do with the H. Difformis, but that's the main change that comes to mind.

The L. Repens trimmed tops would all start to melt after a week of replanting. Sometimes the entire stem would melt. The trimmed old growth bottoms continue to grow fine, as do the other untrimmed tops. See pics. These melted tops were replanted about 9 days ago.

I started to think MAYBE the H. Difformis was sucking up all the nutrients. One by one, I increased all the nutrients I could think of - Nitrogen (more all-in-one), Phosphorus (KH2PO4), Potassium (K2SO4), Iron (DTPA), Calcium (CaSO4), Magnesium (espom salt), and even Manganese (MnSO4). My Nitrate is at 40ppm, Phosphate at 2ppm. I don't have other test kits.

I tried changing light levels. I tried root tabs. But nope, the L. Repens replanted tops kept melting. Something changed in my tank and I can't figure it out. It's killing me. lol

BTW, all the other plants seem to do okay. The only algae I have is green spot algae, which I can't seem to get rid of even at 2PPM Phosphate.

Sorry for the long read. Any advice would be much appreciated.

EDIT: new downsized pictures attached. Previous pics must have exceeded some size limitation. Sorry

 

[Allwissend]

I agree with you it sounds like competition for the nutrients or flow limitation. It seems the photos did not attach, could you send them again please ?

 

[Stigigemla]

I dont think it is the nutrition if the other plants grow. Maybe something is accumulating and the Ludwigidia is more sensitive for that.
How much and often do You change water?

There are IPS tests You can by and send in a water sample to test about 30 different elements. You can check nutrition and all Your trace elements that way but it won't see organic sustances like poison from a plant. The IPS tets costs 20 to 30$ and reef shops sell them.
An active carbon filter will normally bind organic poisons.

 

[Russ S]

I too go through some cyclic behavior with L. Repens. I had some blood-red stems start to take all the light so I cut them off and planted them low and in the back. A week later those cut stems had completely rotted out. So it may be more of a drastic light change than water composition causing the melt. As for the other water parameters, your nitrates do seem a bit high at 40. I run my around 1-2 ppm even though the 2hr guy suggests 5-10. I have become a fan of supplying the macros in the soil and the micros in the water column -- though I am still a long ways away from perfecting that.