Poorly goldfish

[SnipSnapCrashCrack]

Hi,
My two goldfish have been happy in their new 125litre tank for the past couple of weeks but a few days ago my black moor started to stay in the top corner of the tank. He'd swim for a few minutes but always ended up back in his corner.
The past two days he's been hiding under a rock in the tank but comes swimming out happily enough if I go up to the tank and he's still eating whenever food goes in. Occasionally I see him swimming about but mainly he is sitting under the same rock.
I have been doing 30% water changes every day due to a nitrite spike which is finally going down. There is 0 ammonia, nitrite is 0.3 (nutrafin liquid test kit) and the nitrate is going up as the nitrite goes down. It has a filter for 800litres and the fan tail is nearly 2 inches with tail but the black Moore is an inch and doesn't seem to be growing much. I have put aquarium salt in which cheared them up for a day but not now they have 2 air stones too. What's wrong with my fishy ? I'm rather fond of him and just want him to swim around like the fantail.

Ooh and the fantail is swimming all day and looks very healthy. There's no fungus or marks on either fish and the black moor looks in good condition except for his lack of swimming.

I think you're doing everything you can. Keep up with the water changes and daily testing until the tank has been stable and you're sure it's fully cycled for at least a month.

Is there still a chance he might live? He ended up swimming around for a few hours after I wrote this but he's now gone to the bottom of the tank again and is hiding under the rock. I keep thinking he's dead but even touching the tank slightly makes him start swimming about. Starting to wonder if the fantails picking on him a bit if that's even possible I think the Blackmore might actually be getting smaller (even though he does eat) but the fantails growing very quickly. Never thought I could get attached to some fish

Keep the water as pristine as you can with plenty of changes and testing. There's always a chance.

The place we got them from has said to used interpret no.5 General tonic so I've added half a dose of it to the tank. I've done a 60% water change and DP is off to get some plants for them. The fish looks so sad he's had a few moments where he's swam about normally but he can't seem to float he just sinks to the bottom or ends up floating upside down and spinning about. I've ordered some sinking food for them since he used to spend his time at the top looking for food. I don't think he's going to make it though

Don't medicate if you don't know what's wrong. Those general tonics are a waste of money and don't last long if you're doing daily water changes. Water changes will do far more good than any tonic in a partially cycled tank. Ignore the pet shop. They are the ones who sell goldfish in bowls.

Unfortunately black fish died this afternoon, we had placed him in a different tank yesterday as the other fish had started picking on him and he kept trying to swim but couldn't, he was curved round too . He's always been tiny but was getting smaller and never looked well from when we got him so I suppose it was kind of expected. I've added seachams prime to the big tank just in case.

Sorry to hear this. At least you tried your best. It does sound that you had a bad fish to start with, maybe poorly bred stock, or the ammonia just got the better of him/her too soon.

Why are you adding more things to the tank now? Just test, water change, dechlorinate the new water, repeat. Do this until your tank is cycled and don't add any more fish yet.

The water levels were 0 ammonia, 0.1 nitrite and between 5 nitrate in the big tank when he died but it may have been too late to save him. The day he died he was pooing white stringy bits, like a long line.
The gold ones now doing it too and is also at the top corner of the tank just like this one was a few days before he died although the gold one is still swimming round the rest of the tank and eating, he just seems to favour the top corner. Wondering if it's stress or a parasite.
The seachems prime is a water conditioner which apparently makes the nitrite less harmful if used in high amounts. I needed new water conditioner anyway, so have been using that and apparently any nitrite that shows up now won't be harmful as long as I water change every 48 hours. I'm still changing between 30 & 60% per day to try and completely get rid of the nitrite but the test kit says it isn't harmful unless it goes over 0.3
I can see myself loosing another fish very soon.

Ok. White stringy poo can be parasites or just terrible constipation. Feed thawed, frozen peas (pinch off the shells and feed them squashed between your thumb and forefinger).