I started using Melafix for about 4 days now and it hasn't stopped the Popeye I am thinking about using aquarium salt.

Would I be able to use Melafix and Aquarium Salt at the same time?

My Goldfish is in the biggest possible tank I am allowed 20 litre (My mum won't let me have any bigger), he is a common Goldfish measuring about 3 inch's and I won him at a fair. I have had him just over 4 months. I do two 25% Water changes a week and I use Aqua Plus, Waste Control and Cycle on the new water. The water is filtered and has a oxygen pumping through it 24/7. There is plenty of gravel and decorations and I feed the fish pre soaked "Tetra Fin Sticks" twice a day. He is active and has a good apetite.

I also read on the internet about using Aquarium Salt every week to prevent diseases, Would this be ok and not harm my Goldfish?

Any sugestions except upgrading my tank size?
*NO ANSWERS ABOUT UPGRADING TANK SIZE*