Suey and Jay are both right.
The fish are basically pond fish and need biiiiiig tanks.
At the moment their waste is poisoning their water because it's such a small tank so they are suffocating.
They will die in that small tank sooner rather than later. . Like suey says get a bigger container-either a tank or a storage box for now and move the fish and the filter.
You have to have a filter because that is what processes all the waste. I'm guessing the one that came with the tank you have is not going to be powerful enough but it's better than nothing for now.

The shops that sell fish and tanks like this are terrible and it causes so many problems.
But if you're quick, you might be able to save the fish.

And yes. It's definately a toxin problem, and part of the reason you have seen no change is because the tank is so small as soon as you put fresh water in it will be polluted again.
Even in a bigger tank you'll not see improvements for a good week if not more.

Hiya.
The water treatment you need to get,if you haven't already, is something like Nutrafin 'aqua plus'. You add the correct amount to the water you are adding to the tank and it removes chlorine and chloramine from the tap water. It is normally in the section with the medicines.
The test kit you're best getting is the API liquid dropper one-it has test tubes in it and little bottles of solution for the different things you're testing for. It covers all the important ones like ammonia, nitrate and nitrite.
The shop will probably have some packs of paper strips that test the water. These are cheaper-but quite innaccurate so don't bother with these.

If you do a search on here for fishless cycling you'll see that it takes about 4-6weeks of having a tank set up and feeding it with some form of ammonia for the bacteria to build up.
There are links on here to the bacteria cycles too that keep your filter healthy.
You'll need to do a fish-in cycle now as you have fish, which will involve a lot of water changes and testing of the water until the filter bacteria can convert the ammonia from the fish waste to nitrite and then to nitrate. Once your tank is cycled you'll then be able to get rid of the nitrate with your weekly water change.
If you haven't got one already (I'm typing this on an iphone&cant check back to read your original post), then get a gravel Hoover whilst you're at the pet store as your gravel will need hoovering to get rid of the fish poo.

And one last thing-from your picture your tank does look too small for the fish...if you know the volume of it we'll know for sure, or post the measurements and we can work it out.
You'll really need to be looking at something like 120l minimum for 2 goldfish as a starter size as they do get big!!

And one last thing, when you say you change the filters each week, what do you mean? The bacteria lives in the filter, so the filter sponges just need a squeeze out in old tank water when you do a weekly water change to stop them from getting clogged up. If you put new ones in all the time then it'll be needlessly expensive as well as getting rid of bacteria.
Oh, and why only one fish sick? The other one will be sick but may not be showing symptoms yet. Remember, in the wild fish are prey so will not show signs of sickness till they are really ill.
HTH for now.
Have fun shopping.
Andi.
PS. Don't let you pet store sell you medicines etc. The problem here seems to be the fish in an uncycled tank. Get the water right and the fish will be a lot happier.

Hi Jason,
just a thought... If you don't get any interest in someone giving the biorb a good home, then you could try advertising it in your local paper.
I ended up doing just this after not being able to find a home for a 30l biorb I'd been given &it sold straight away. I put the money towards some fish food for all my fish.
Yours is bigger though so you might have more luck.
Andi x

Thankyou violet. I will.

P@H plants can look a bit sad if they're not freshly stocked.
I've got lucky many times and got there when the plant tanks are full of plants and got some great plants from them.
Never had a problem with disease from them.
Guess it depends on which p@h as there's one store a little further away than my local one and there's no way I'd buy anything living from there-it's filthy and everything looks sick.
My goldies don't eat amazon swords too fast, so I have quite a few of them planted.

Hi Suey, I'm gonna have another look tonight.
I was really tired last night and as I want several foods I was getting all mixed up with what seller does them at what price.
If I get them from seperate places I might just as well go to the store in Enfield and get them in person by the time I've paid several p&p.

I'll see again tonight, but I may have a trip out next week after work one afternoon.

just bamboozling myself on there now.

Hi guys..
does anyone know the cheapest place online to buy Hikari goldfish food?
I could go out and buy some, but the store is a drive away and I'm on a spring cleaning mission this week, so I thought I'd get some delivered.

Andi x

I just wondered. I cant access it from my phone, and whenever I'm on the computer it is an empty, draughty room.. I wondered if I was missing a party in there when I'm on my phone! lol

Whenever I've been on the computer as opposed to posting on my phone, the chat room is empty.
Does it ever get used?