Your tank is going green cos it probably has too much light on it. It appears to be right next to a window plus you have white gravel which will make the situation worse as it reflects even more light.
Shift the tank away from the window and change the gravel for something dark (it doesn't show up the algae so much either )
This size is ok for your goldfish while small but they grow and the tank will be way too small. I really hate the idea of just returning fish to a shop when they get too big and swapping them for something smaller each time. If you are going to keep a live animal in artificial surroundings, totally dependent on you for its life, you should make sure they are in the best you can get and most suited to the animal concerned.
Lots more live plants will compete with the algae and hopefully starve it of nutrients so it dies off as the plants mature. Fast growing plants like hygrophila are good (often sold as "bunch plants" in lfs). Search the net for a pic of it so you know what you are looking for.
Plastic plants will not stop the algae by the way, in fact it will get worse.
Go down to your local waterway and get some oxygen weed. Free and good for the fish. Check it is algae free, try to pick it from reasonably fast flowing water.
A pleco will grow way too big for that tank. Bristlenoses are like little plecs but none will eat slimey algae.
To make it tropical you just need a heater as you have everything else.
Do not add any more fish to that tank as the problem will worsen - more fish - more poo - more nutrients for the algae.
I know people have been known to have plecs and goldfish together but in bigger, tropical tanks. The fancy goldfish prefer it a little warmer than the common ones.
If you go tropical, what about a single Siamese fighter and a couple of bristlenose catfish?