Plant Leaves Discoloured

I have a 54L tank with 2X anubias and 1X java fern both tied with thread to rocks/wood, as well as one water sprite (floating) and some neon tetra & corydoras. I added the plants a few weeks ago and have since noticed the leaves becoming quite discoloured (dark spots) especially on the java fern. As I am fairly new to this I was wondering if this is normal or if I need to do something differently?
I use a fertiliser that I add once per week (EasyLife Pro Fito), and also add liquid carbon once per day (EasyCarbo).

The brown stuff is just algae. When the water sprite grows and spreads it will reduce the algae.

I wouldn't add carbon. There is heaps in the atmosphere (carbon dioxide) and you don't need it for those plants.

You need a picture or something on the back of the tank to make the fish feel more secure. You can buy pictures from pet shops or use a plastic rubbish bin liner, a bit of coloured card or even a newspaper. Just sticky tape it to the outside on the back of the tank.