I've just added 8 Cardinal Tetras to my tank and they've been fine. However, one of them seems to be just hanging there and not swimming about with the others. It's dorsal fin and tail were "quivering" in the beginning but this seems to have stopped now.
Have I got an ill fish or is it just the change of environment and it'll be ok?

Can you post more about your setup and your water test results? The advisers can help you better with more information.

I took readings this morning and all results were OK. Tanks been up and running around 1 month and I've had 6 Zebra Danios in there for just over 2 weeks.

I added the Cardinals and 2 Corydoras and they all seem fine except for this one tetra.

I assume this is a different tank to the one you posted about recently which had just been set up (Rio 125, 10 Zebra Danios - lid wrong way round)?

Can you post tank size, test results for ammonia, nitrite nitrate and pH and tank temperature for the tank the Cardinals are in please? Did you cycle the tank before the first fish went in and if so how?

That's the same tank, though it's 6 Zebra Danios. I did a fish in cycle and ammonia is zero. I'm doing a 25% water change every 2 days (cleaning gravel) with hand vacuum.
I've been adding dechlorinator in the recommended amounts 're each time. I added bacteria growth speed up in the beginning.

Ph is 7.
Kh is 8.
Gh is 10/11.
NO3 is under 20mg/ltr.
NO2 is 1 per ltr.

It's fin and tail have actually stopped quivering as much now but it's still on it's own.

Hope this makes sense. I'm a bit puzzled that the other 7 are looking OK.

That's the same tank, though it's 6 Zebra Danios. I did a fish in cycle and ammonia is zero. I'm doing a 25% water change every 2 days (cleaning gravel) with hand vacuum.
I've been adding dechlorinator in the recommended amounts 're each time. I added bacteria growth speed up in the beginning.

Ph is 7.
Kh is 8.
Gh is 10/11.
NO3 is under 20mg/ltr.
NO2 is 1 per ltr.

It's fin and tail have actually stopped quivering as much now but it's still on it's own.

Hope this makes sense. I'm a bit puzzled that the other 7 are looking OK.

Water temp is 25.

Hi

If nitrite is 1ppm then your fish are at serious risk. Nitrite is highly toxic and that is part of the explanation for the problem. Fish in cycling isn't recommended these days because of the risks to fish involved, but if you've ended up doing one, you need to change enough water every day to maintain less than 0.25ppm ammonia/nitrite in the water. Have a look at this article for details. If you switch to using Seachem Prime in a double dose for the whole tank at each water change as your conditioner, this will detoxify any residual ammonia and nitrite between water changes and help protect the fish.

Are you by any chance using paper test strips? These are not very accurate and a liquid test kit is better ( especially during a fish in cycle). Your kH of 8 isn't really compatible with a pH of 7 so there's something off there, which is why I though you may be using strips. If the kH and gH are any where near accurate, the water is too hard and alkaline for Cardinals so they won't do well long term in your water as they need soft acidic conditions.



EDIT Just seen it's 25 - that's too warm for the Danios and a bit cool for the Cardinals. These two species really aren't compatible as Danios are not true tropical fish. Danios do best in tanks at room temperature between 18C and 24C.

Nitrite (NO2) is just as bad as ammonia to be honest, the reading for that should be 0, so your results aren't actually fine at all.
Fish in cycles are not recommended as the damage to the fish may not show immediately. As you have added fish to an immature tank there is no way of deciding if the strange behaviour is that fish being more sensitive to the water quality issues. If you are having to do water changes every other day to keep the fish safe they really shouldn't be in there yet.
Cories should be in larger groups than two, but you shouldn't add any more fish for a while yet, you should really have been a little more patient and waited til the filter had matured enough to cope with the danios before adding any more fish.
Also danios and cardinals do not appreciate the same water conditions, danios prefer harder water and cardinals prefer softer water, as your water appears to be quite hard, I would suggest returning the cardinals and the cories too as they also prefer softer water, and stick to species that will be happy long term in your water.

Jesus! Why do these shops not give correct advice?