It will be a combination of light and co2 - too much light for the amount of co2 getting to the plants. Reducing the light will definitely help but look at getting the best distribution of flow also. You could move the filter intake to the same end as the lily pipe. That way the flow should go across the top of the tank and then return along the bottom through all those low plants. It looks like it probably just flows right across the top of them at the moment. That should get more of the co2 to where it is really needed. For a couple of days, put your drop checker low down on the front of the glass as close to the melting plants as possible to see what it's doing down there.
The flow seems like it is going where it should, right across the top aided by a powerhead and then down the glass at the far end and back along the bottom and up again at the lily pipe/powerhead. I've got the light sitting on the edge of the glass so space is limited to each corner of the tank for anything to overhang the glass. Found 2 shrimp on the floor this morning, but they where still alive so I presume they had just jumped before I came down. Managed to get them back in and they seem happy enough, still doesn't stop me worrying everytime I leave the house now or that the dogs might get them before I do!