@fcmf Yes they can be difficult and frustrating, IMO water and plants are the tricky things in fishkeeping.
I moved the entire contents of my 200 litre tank into a bigger one at the back end of last year, the only thing that changed was part of the substrate [It was gravel and I now have plant root balls in their original gravel with sand filling the gaps and lightly covering the exposed gravel] However it has taken me the best part of 6 months to settle things down again and getting the plants growing or even not dying.
I almost lost all my Water lettuce, I was down to just a dozen sick plants, and remember I was trying to give this away when it was in the old tank.
I had a green hair algae outbreak that covered and nearly killed a Cryptocoryne forest that had spread to half the old tank. The Anubias was covered in black beard algae and the amazon sword leaves were dissolving and had both algae.
All my filters were choked with dissolving plant matter each week and this added to the maintenance work. This was with the same water, plants and idiot running the system.
The fix was patience and changing single things until I got back to a stable condition. The lights were the first, they were initially completely underpowered so I replaced one and then had too much light... I dialled back the photo period. I have stabilised the yeast CO2 with a second culture bottle and alternate biweekly changes (almost suffocated the fish with a CO2 overdose once) so I now have a air stone running when the lights are off as a temporary measure while I make a new CO2 reactor that will allow me to use my yeast CO2 regulator again and can switch the CO2 injection off during the night. This is all aimed at stable CO2 levels.
Finally I started dosing fertiliser twice weekly (half dose) and have gradually found the correct level.
So don't give up, measure the water parameters (KH and pH) and find out what CO2 level your water has - even if you don't want to add CO2 you need to know how much Carbon is naturally in the system. Check Phosphate levels (look longingly at FKS Coatbridge's small selection of plants, smile sweetly and see if they will do the tests for you
) Have a look at the light spec and see if you can work out the Watts/litre (not a very useful number but it will give a clue if its is light that is needed) If you are not adding fertiliser try a small amount or perhaps add root tabs close to the plant.
If you add the results and comment on the effects to this thread over the weeks (it will be weeks I'm sure) you will get lots of other thoughts as well.
Chin-up and keep planting! If you would like some Water lettuce or Amazonian swords I now have loads again