Help! Everything in my tank keeps dying after getting cardinal tetras (TW - Dead fish pic)

[lukesfishies]

Should I put the guppies in saltwater long term too or will the 15 mins or so in strong saltwater be enough?

 

[Retired Viking]

This did occur to me last night after we seemed to have ruled everything else. Sorry I had to sleep and I'm only just in from work.
Since you (@lukesfishies) are in the UK antibiotics are not available without a vet's prescription. An alternative solution would be to add 1 heaped tablespoon of aquarium salt (not marine salt you just want plain NaCl with no additives). Keep it in the tank for two weeks. Do your regular water changes and replace the correct amount of salt for the water you replace. After 2 weeks just continue water changes as normal.

Edit: Sorry that's one tablespoon per 10 litres of water.

I would follow these directions from @seangee since this appears to be an internal not external issue.

 

[lukesfishies]

Ok so we lost two more guppies overnight. I'm treating daily with melafix. I did the saltwater dip yesterday, but I'm not sure how a long term saltwater tank would work, would it be worth it doing it only for the guppies, since the tetras can't handle not having a heater? We only have 3 guppies left now.
Going to ring the lfs today and see if they know anything

 

[lukesfishies]

We've just been to the LFS and the guy confirmed it was a bacterial infection after seeing the pics. He gave us myxazin to treat them for 5 days, and hopefully it works! One of the loaches has disappeared too so hopefully it's not dead

 

[Byron]

Something that has not been mentioned yet (unless I missed it)...what about the remaining cardinal tetras in the store tank? Have any of those died? It would seem, though not a certainty, that the cardinals introduced whatever this is, so the state of those in the store tank would be helpful to know.

I am not at all convinced this is bactyerial; the description you initially posted would not suggest external bacterial issues, but internal protozoan or similar. Signs of a bacterial infection on dead fish is a very different thing, but not necessarily the initial issue.

 

[lukesfishies]

Something that has not been mentioned yet (unless I missed it)...what about the remaining cardinal tetras in the store tank? Have any of those died? It would seem, though not a certainty, that the cardinals introduced whatever this is, so the state of those in the store tank would be helpful to know.

I am not at all convinced this is bactyerial; the description you initially posted would not suggest external bacterial issues, but internal protozoan or similar. Signs of a bacterial infection on dead fish is a very different thing, but not necessarily the initial issue.

We asked them if they had any issues with the other tetras and they said they didn't know of any, but that it might come from shock.