new fish dead red blisters now other fish red gills

Question
I have gold fish. I Moved to bigger tank used same filter fluval 3plus and 1/2 of existing water in new tank which is 60l (UK. I do a 20% water change every 3 days.

Well where to start i bought a bigger tank and also bought 3 yellow fish to. They where all fine only i think i did a mistake as there was one yellow fish that had a red streak through its body which so i thought was the colour. Brought them home and a few days later i noticed a white kind of blister on one of the yellow fish tail not like cotton wool though. I treated the tank with disease free (king british. 3 days later woke up to find it dead and all underneath its body where red blotches. NOW all my other fish are up at the surface gulping at the air and 3 of my fish have redened gills. I thought could be ammonia spike with putting in new fish so done 50% water change.

I know the tank has been disrupted so these readings are false. I just dunno what to do all i have had since i got this tank is trouble.

Ammonia- .25 (used to be 0 when the little fantail was in it)
Nitrite- 1.0
Nitrate- 10
Ph (tank)- 7.4  

Answer
Hi Tracey;

Make a 25% to 50% water change every day until they feel better and also add aquarium salt. Your fish store should have some. Follow the instructions for dosage on the label. Nitrite poisoning causes the fishes' blood chemistry to change so their blood can't carry oxygen very well right now. The aquarium salt reverses some of those effects. It also helps to add bubbles from an airstone hooked to an air pump to increase the oxygen level and help the whole system work better.

The problem you have is that the tank is just too small for so many goldfish and too many were added at one time in such a new system. If it's going through the break-in period the ammonia and nitrite will rise even without adding additional fish. Beneficial bacteria that breaks down the ammonia and nitrite needs to grow larger colonies to accommodate the new fish. If too many fish go in all at once, the beneficial bacteria can't grow fast enough and the toxins build up to lethal levels.

Each goldfish needs at least 40 liters all to itself. They are very messy fish that get BIG and they just need lots of space to thrive. When adding new fish you have to do so very slowly, one per week until the tank is at the safe population limit, but your tank was already almost at it's limit and it was too newly set up too so the whole thing just became a disaster. It's also possible the one fish brought in an infection. Just do what you can to get the ammonia and nitrite levels under control by doing water changes and using the salt. Hopefully it will get better very soon, but don't add any more fish. It's already overstocked.

I hope things get better soon...

At Your Service;
Chris Robbins