Lower leaves dying

[bridgey_c]

I have got a 30l low tech cube. It has been set up about 3 months now and I have been amazed at the growth levels. I even have some UG holding its own at the front of the tank. I wouldnt say it is thriving but it has lasted a few months now without disintegrating! Lilaeopsis is showing plenty of new growth, crypts and anubias doing well. The growth rates of the ludwigia repens has been astounding but in the last week or so its lower leaves have started to turn yellow and a couple of the stems have lost their lower leaves. The new leaves are fine but the yellowness is moving upwards. The majority of the stems are fine at the moment but I can tell not ideal. The flow is great with the little internal fluval and spray bar and I can only guess its a carbon deficiency? The lights are only on 6hrs a day and I want to keep this a low tech tank so I dont want to go down the easycarbo route.. (I have enough work with my other tanks)...

I drop a bit of my EI liquid in every so often but its not routine. Is my only option to reduce the lighting time even further? Or are there any floating plants that dont mind a strong surface current? Or is this a nitrogen deficiency? ( I have tried increasing the dosing the last few days but its prob not had enough time to affect the plants yet). Cheers