Keeping 2 goldfish in a 2.5 gallon tank is equivalent to keeping them in a goldfish bowl .. 1 gallon per goldfish.  I suggest you keep upgrade to a 29 gallon as soon as possible, because toxic wastes can build up very fast in a 2.5 gallon tank.   Your goldfish should show some improvements once you move them into a proper sized tank.  

You should change 50-60 percent of the water immediately and feed your goldfish only 1 time every 1-2 days.

It doesn't sound promising. I agree that you need more room for your fish. They are much happier when they have space. I have 2 sick fish at the moment and have them in a 40 litre container. It's all I can do until they get back on their fins. Yours sounds like an ammonia issue to me. You absolutely must buy an ammonia test kit and test every day. Ammonia will kill fish quickly! Also, I don't know that I'd be changing the filter so regularly. You need bacteria to build up in there because they remove ammonia from the tank. I'd wait until the media is quite filthy, and then only rinse it in some of your tank water. It doesn't need to be spotlessly clean because you require good bacteria to help your system. Can you post a photo of your set up? Good luck.
Hope this helps.

I'm sorry to hear about your fish. If your fish is really suffering and you know it's going to die, I wouldn't make it agonize any longer in vain and just put it to sleep by euthanizing it in the freezer. The remaining other fish I would take back to the store like Nossie said, unless you are willing to find it a larger home...even a cheap 55 litre tupperware container would due until you can afford a tank!