Can anyone help me?
I bought a 20litre tank from pets at home and have had nothing but problems since. I am aware now that it is small, so since my last fish died, i cleared the tank completely out and chucked away everything. I bought new gravel, filter, live plants etc, and had the water tested last week at the local pet shop to ensure it was fine. He advised me to only get one fish so i got a fancy goldfish, and popped him in the tank. ONLY 3 DAYS LATER HE HAD WHITE SPOTS EVERYWHERE!!!  I went back to the shop with him, and the guy confirmed it was white spot and sold me some malachite green stuff to start treating him with. I was advised to take out the middle sponge of my filter as it has carbon in, and just put some filter sponge in, and give the treatment every 48hrs. Since then i have done this on mon, wed, fri and today- sunday, BUT ITS GETTING WORSE! He's totally coated in white, i can see the blighters on the inside walls of my tank, and the fish has stopped swimming and is lying with fins down at the bottom. he still eats his food every other day as soon as i drop it in, but i'm worried hes going to die, and must be suffering terribly.
I really have tried hard to make the environment right before i got a fish, as i know water quality is vital, but now i'm so disappointed and frustrated!
Can i do anything else to help him? It says to do a 10% water change after 5 treatments so will do that on Tuesday if hes still alive. 
Any advice??

I don't know about Malachite green, I've never had a fish with white spot! But Ich is a parasite, how about adding a small concentration of medical aquarium salt? Have a look in your pet shop and follow the dosage on the package.
Are you changing water regularly while treating? If not, your fish will die anyway living in the bad water.

I always used a kind of salt made by the German company Sera, it's called "ectopur" and is great to use against fungus as well, and it won't meddle with the filter in any way