What's killing my Cardinals?

[k1rr1e]

I have six Cardinals in a community tank with other peaceful fish.

Yesterday I introduced two more and this morning both of them were dead. They were fine last night, active and eating, but were floating in the tank this morning, both with most of their tails bitten off. The two new arrivals were slightly smaller, hence how I know it is these which have succumbed.

The only other inhabitants of the tank are female Gourami's x 2, Corry's, Rummy Noses, Oto's and Yamato Shrimps. My tank water is fine; no nitrates, nitrites or ammonia and all the other fish are good. Water temp is 28C, PH 7.2.

I suspect that these losses may be stress related and the tails have been nibbled post-mortem. Am I correct?

Best regards,
Eric

[Carylnz]

That is quite possible. Another reason to quarantine new fish. Doing so would have helped give you an answer.
It is possible they were still getting used to their new home and water parameters when the current residents started bullying them to make sure they knew they were at the bottom of the pecking order. The combination could do them in.
How long has the tank been set up? It is unusual to have a mature tank register no nitrates.

[k1rr1e]

The tank has been set up for around three months.

I tested a water sample again last night using a proper test kit rather than test strips and the nitrate reading was 20; all the other water parameters were as per my mail above. I returned the two dead fish to the shop yesterday and they were also happy with my aquarium water quality.

I have lost another Cardinal this morning - its showed no outward signs of disease, virus, fungus etc and this fish was one of the established residents, so I am at a loss to explain what is going on as all my other fish and shrimps in this tank are fine... I currently only have three cardinals remaining; so are there too few of them?

Best regards,
Eric

[Carylnz]

Cardinals do like to be in a bigger group but not being so does not usually kill them.
Do you know if the store from where they were purchased has lost any fish too?
Was the new dead cardinal the smallest of the group remaining? The bigger ones may have developed a taste for bullying.

[k1rr1e]

Down to two remaining now, there was another dead one in the tank last night.

I would say that they were all of a similar size, apart from the two new ones I purchased earlier in the week, which were slightly smaller.

All other tank occupants still absolutely fine!

[Carylnz]

I wonder if the new ones had something wrong with them and it has spread to the others. Perhaps that species is more susceptible to whatever it is which is why they are dying and the rest seem fine.

[k1rr1e]

Last two cardinals were dead this morning, so I've now lost eight in total.

Water quality is still good, if a little cloudy and all my other fish and shrimps in this particular tank are fine.