My fish is not well but I can't work out what's wrong. Can anyone help? :(

He is a goldfish (well, he's gone silver but he's a goldfish really). His dorsal fin is always down, flat on his back, which I've heard is not a good sign. Also sometimes he goes weird and just floats (sometimes even upside down... very worrying!) He shares the tank with another fish that we got at the same time, but for some reason that fish is now three times bigger than mine. He doesn't show much interest in food either (which probably explains the size difference tbh as the other fish is a bit of a pig)

Any ideas?

AN UPDATE:

He seems more alert today and his dorsal fin is at half mast (an improvement?) But there are half a dozen red marks, like pinpricks, on his left side and another, larger mark under his jaw. He's still 'going weird' but he keeps jerking suddenly and is acting kinda frantic.

I'm really worried... Is he ill? :unsure:

ANOTHER UPDATE:

Stuck at work all day so couldn't keep an eye on my fish. I came home and was told that he had to be taken out of the tank cos he was bleeding. There are loads of spots down both his sides and his left fin is severely bloodshot. He's ben put in a bucket by himself (much to the other fish's annoyance) as it turns out that Mum (who cleaned the tank yesteday morning) used a new Tetrafin water purifier instead of the usual tonic salts.

At first I didn't think it could be the water cos the other fish is unaffected. Is it just that my fish (who's called Dylan, btw) is allergic to Tetrafin? :unsure:

At this point I'd appreciate a simple yes or no; I'm quite fond of Dylan and I'm very worried.

It sounds like Dylan has septicemia which is when you can see the blood inside the fish in veins. Since he had problems before the new water purifier, that's probably not the cause of the problems. The water change was very stressful on him when he was already sick. Being in a bucket will make him get worse. Why did your mother think he should come out the tank? If it was just because he was bleeding, that wouldn't harm the other goldfish but that fish might pick on him more.

It's sounds like Dylan is very sick. He may not make it. I suggest putting him into another tank if you have it. Otherwise, back where he was. Add some aquarium salt and antibiotics. Be sure there is good aeration. Check the tank's pH, ammonia, and nitrite to be sure those are ok. How big are the two goldfish? How big is the tank? How long have you had him?

I'm sorry to give you more questions than answers. It's hard to diagnose fish even when they're your own let alone from a few sentences. I hope Dylan beats the odds and improves!

We've only got the one tank, i think its four gallons (it's very old, older than me!)Dylan is three inches long and the other is just over four inche but much fatter. We've had them about 18 months.

Mum thought it was the water cos the red spots and bleeding only happened after the water change. He won't be in the bucket for long, only while she changes the water again.

He hasn't got any worse whilst in the bucket though, so here's hoping! He ate all his food too.

I'll keep you posted. :unsure:

UPDATE:

Great news! Dylan seems to be feeling much better today. he is still very red but nowhere near as much as he was last night. you should have seen him, poor thing :(

But today he's swimming around, eating very well and his fin's no longr flat to his back. I haven't seen him poo yet so I can't tell if their still white and stringy like before but he seems so much better. I know its early days so fingers crossed he keeps it up.

Will keep you posted :)

I'm glad he's better!