Sad goldfish

Question
Oh! Where do I start! We had a tank set up for a week before buying two small
goldfish - then the tank sprang a leak within 2-3 days so we had to hurridly
transfer our fish to a plastic storage box (one fish became listless at this
point). We then acquired a new 25 litre tank which we set up and left for a
week. We had the water in the new tank tested at PetSmart/PetWorld (big pet
store) and were told the water was good for our Goldfish but to mix 50%  of
their old water in with the new - which we did. The listless fish continued to
mope around and look sad - the other fish seemed fine. Neither fish have
ever eaten much and I clean up the excess food daily. The listless fish died 4
days ago and now the other fish just sits at the bottom of the tank.
Occassionally he will swim around, but only for a short time. Sometimes he
will swim up to where the filter chucks out the clean water and swims against
that bit. Since the other fish died I've done 2 50% water changes (with treated
tap water) but he doesn't improve.

I realise he's had a lot of swapping and changing with the tank but apart from
50% water changes what can I do. He's not eating at all now.

The tank is 25 litres, with gravel at the bottom, an ornament (bought at the
pet store) and a filter which pushes the water back onto the surface of the
tank creating bubbles.

Hope I've given you even info! Looking forward to hearing your advice.

Answer
Hello Nicola,
Yes you have given me quite a bit of information, and thanks for that.
I have bad news though, a six gallon tank (25 liters) isn't going to cut it for your  goldfish. Yes, it does stress them out to be moved around a bit, that's why we recommend not moving your fish unless you positively have to (which I realize in your case it was necessary). But that may have been the reason why your listless goldfish started to degrade in health.
But a 20 gallon tank would really be a much better size for a goldfish. The reason being is that they grow to at least 6 inches long, and are very large bodied. They simply produce too much waste and need much more swimming room than anything smaller than 20 gallons can provide. Bigger is always better, when it comes to tank size.
I would upgrade to a larger tank as soon as possible, and start adding aquarium salt to the tank. It will help him recover much quicker, as well and possibly prevent disease.