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

Are you able to see whether the remaining 3 fish have full-length barbels? (Compare with pictures of bronze cories' barbels online.) If they don't, then it's likely the gravel is wearing them down and ultimately leading to their demise, in which case I wouldn't get any more cories unless you change the substrate to sand.
A few other possibilities:
* Did you quarantine the new cories for a month or so? I'm reading it that you possibly didn't, in which case their deaths could possibly be accounted for either by the substrate wearing down their barbels or them succumbing to something in the tank.
( Do you know if your water softness is very different from the LFS's water softness? (Might be worth checking out the LFS's postcode on the water supplier website and/or finding out if they harden the water in some of their tanks. It might well be that the recent ones succumbed either to a sudden change in water softness / PH - or possibly just couldn't cope with the stress of transit from wherever > LFS > your home.

I wouldn't get any more bronze cories for now. I also have very soft water but didn't have success with pygmy cories. If I were you, I'd wait for a few months (in the hope that anything in the tank might dissipate), then possibly consider a different type of cory if you're particularly keen on them and prepared to change the substrate to give them a better chance (or buying bronze cories at a different time of year when the stock might be better quality).

Just to add, one of them did have black barbels after it died (that was the one that died 2 days after purchase). Not sure if that is a sign of an infection or damage.

Thanks for the replies.

I have attached a photo of the sand in the 80l aquarium, and also of the barbels on one of the corys.

I guess I am wondering if the barbels have been damaged by the gravel in my aquarium (I do wonder if some of them died of stress), and if so, would the 80l aquarium be more suitable for them?

My main concerns are potentially introducing contagious diseases into the 80litre, stressing out the corys by moving them, and whether they would get along with a bristlenose pleco.

I have corys in every tank that has plecs and they seem to be ok together. My BNs tend to be quite solitary, and the corys are more outgoing, so they tend to use different parts of the tanks.  All my tanks have sand substrate.

Saying that, if you are planning to move your corys I'd be tempted to quarantine them in another tank before risking them in with your BN.

I think you may have a tricky decision head of you when you consider leaving the corys on gravel or moving them to the other tank. Whatever discussion happen here on the forum regarding the pros & cons of each option, the final decision is yours and we will support and assist however we can. 

I agree that your corys would be better on sand long term.
I still think you may have been unlucky with your corys. The ones I have are generally quite sturdy.
Let's see what the others think and perhaps we can come up with some options for you. 

I've set up my quarantine tank twice in the last 12 months.

The first time I used media from my main tank, and just to make sure i added enough ammonia to get 3 ppm. Next day it was still 3 ppm. A week later it was still 3 ppm. I ended up doing a 7 week fishless cycle before i could get fish. The main tank has a lot of plants, mainly slow growers but the surface is covered with floating plants which were water sprite at the time. Presumable there are very few bacteria in the filter media because of the plants.

The second time, I didn't bother with mature media. Instead, I moved 2 large water sprite floating plants and bought 2 bunches of elodea when I went fish shopping. Daily tests showed no trace of ammonia or nitrite in a 25 litre tank with 12 kuhli loaches.

Rather than the plants containing bacteria it is more that their photosynthesis utilises the ammonia excreted by the fish. As such it is important to ensure the quarantine tank is lighted if you go down this route as the plant will need the light to convert the ammonia from the fish into energy for their growth.

 I do think that many underestimate the power of plants for filtration.  Whilst flow is still needed in a health tank, I do sometimes wonder if my plants might not actually grow better if I removed my filter media from my filter as they prefer ammonia over the nitrate added in plant fertilisers.

That's good to know that plants can remove ammonia, without converting it to nitrite.  The nitrite stage of the cycle always seems to take the longest!

I do sometimes wonder if my plants might not actually grow better if I removed my filter media from my filter as they prefer ammonia over the nitrate added in plant fertilisers.

Hi @Matt

I did an experiment recently to check this out. And I hope to repeat the experiment soon. In short, I found that the plants (your plants, in fact) did not appear to take up the ammonium. Instead, nitrifying bacteria seemed to beat them to it - even with no filter! If you're interested, you can read more on t'other forum. You'll know the one I mean.

John

I would guess the problem is the gravel. Gravel of any kind is never ok for Corys. It wears away the skin on the barbels causing sites for bacterial attack.

Also ime a substrate change is not very stressful for fish if done carefully while the fish are in the tank. Scoop it out with a margarine container, replace with a margarine container. Start at one end and work your way across. The fish are usually very interested in what’s going on....