How to achieve thriving plants in a goldfish aquarium?

Are you an experienced goldfish keeper, with a keen interest in creating a natural environment within your aquarium? If you are, I would love for you to share some knowledge!

I will give you all a bit of a description of my current set up and what I would like to achieve:

I have a 200L goldfish tank. I only have one goldfish in the tank, due to the fact that I have discovered that in the context of goldfish, less is definitely more. I also have some cold water minnows in the tank.

I don’t have a problem with the fish over eating the plants, and I have two grow tube bulbs in the tank for lighting. I have a bubble tube which extends across the width of the back of the tank for aeration. I also have a great external canister filter, and the water quality is optimal.

If anyone experienced with growing plants in a cold water tank could let me in on a few tips on their favourite plant varieties (my java ferns and pond weeds look the healthiest, the least healthy being the amazon swords), fertilisation methods, etc would be appreciated. I would love to add some sort of more nutritious substrate also. Sadly I planted these live plants after my tank had been set up and cycled, so I only have standard aquarium gravel. Seeing as the tank isn’t empty, I am unsure about the best way to go about adding extra substrate (worried about possibly making it muddy or unsafe for my fish if I just added it now).

Ideally, I would love to create a very green, ‘lush’ aquarium. I think the long, wispy background plants are beautiful, however I am not sure that those would do well in a cold water tank. I have seen photos with aquarium ‘grass’, especially in the foreground. I do have moss, but have not had any luck with any floor coverage.