Bronze Corys Keep Dying, Bad Stock Or Am I Doing Anything Wrong?

I do have a tank already with a sand substrate (80 litre with a bristlenose pleco and some black neons) which I could move them into. It does have a few rocks in there (I use them to create a barrier between the filter and sand), would this be a problem?

However I am worried just in case it is not the gravel and they are infected with something.

I am thinking of setting up a small tank and putting the last 2 corys in there (I think around it's 12 to 15 litres). I would fill it with water from the main tank and adding some floating plants. Hopefully these will remove ammonia as suggested in the other comments, but not sure how I could provide enough light for these, would a desk lamp over the tank work?

I would keep this tank bare at the bottom. I will try the small filter I have (healthy fish have been fine with it in the past, but it's too strong for weak sick fish), and if it's too strong, use an airstone instead or look at getting another filter.

If they are still alive in a month and appear healthy, I would move them to the sand substrate tank.

What are everyones thoughts on this?

Thanks

I think it's worth a try putting the last two into the smaller tank with some sand.
Move some filer media from the main tank and put it in to the small filter.
See how it goes with a desk lamp for plants. It is something I've done short term and the plants survived.
Good luck, I'm keeping my fingers crossed for you and the corys.

Thanks!

I moved them into the small tank about half an hour ago. I found a small LED spot light for a display cabinet which I am using for the plants. I put some filter media from an existing tank into the small filter, and the filter flow doesn't seem to be bothering the corys. They seem quite relaxed now in the small tank, so hopefully being caught and moved hasn't been too stressful for them.

Thanks for the filter recommendations. I have swapped to a sponge filter for now as I felt the filter I was using was too strong (the corys became alot more active after I removed it and started digging in the sand), but I have still kept the filter media in the tank. I will see how they get on with the sponge filter for now, and look into getting a new filter at some point.

Corys are still alive in the small tank, but having trouble maintaining good water quality. I did not feed them on the day I moved them, or yesterday, but ammonia and nitrite have now climbed up to 0.25. I am doing a daily water change and adding Prime. I also added more filter media from my Betta tank and some gravel from the original aquarium (I put it in a filter bag) to try and help, but it doesn't seem to have done anything yet.

Glad to hear that your corys are doing well in the other tank. 

It looks much smaller now, wondering if it is an Ich that has burst or just debris which happened to be stuck to the fish for a while. I may leave it another week or 2 before putting them in the 80 litre aquarium to be on the safe side!

Unfortunately I lost one of the corys and the other looks like it isn't going to make it.

Everything went well in quarantine, and I moved them into my other aquarium 3 days ago. I suspected that the white dots were either sand or possibly residue from Prime I was putting in the water and they seemed happy and healthy until I moved them. They seemed OK in the main aquarium, they hid alot but were still eating and scavenging in the sand.

However this evening I went to check on them and one was upside down looking quite bloated. It was dead. The other one looks like it has possible buoyancy problems too and I suspect it may be dead by the morning.

There have been no ammonia or nitrite spikes and I acclimatised them slowly.  I removed all rocks and anything they might hurt their barbells on in advance, so it definitely wasn't that. All I can think of is that it is stress.

The only 2 variables that were different were the tank mates and the filtration, so the increased flow rate is definitely a possibility. I was using a very gentle sponge filter in the quarantine tank and in the 80 litre it was a large internal filter which created a bit of current. I am tempted to try a sponge filter in my main aquarium as the water flow does seem to stress some fish out and small fish do get stuck behind the filter from time to time!

I think I will have to give up on keeping cories, they seem far too sensitive!

Unfortunately the other cory died today

I thought corys were hardy and easy to keep fish when I bought them. I even went for bronze over panda or albino as I thought they would be hardier. I don't think I will risk getting any more as I have no idea what killed them.

Sad news about the second cory.
I can't put my finger on what the problem could be either.