Dead Cory's! Help!

[Cardinal]

I have a fairly new 125 litre tank (2 months old) containing 10 Cardinal Tetras, 5 Pearl Danios, two small corys and I also had two larger corys. The two larger corys came from the same shop and I lost the first one after two weeks, it appeared to get some damage to the gill area and within about 2 hours of me noticing this (it was fine before this) it went noticably pale, flopped over and then died! I presumed it was damage caused by a graze or something and as I didn't even have time to think about treating the fish, I just put it down to bad luck. However this morning the exact same thing happened with the remaining larger cory, he was fine last night and then we noticed this morning that one of his gills was a bit red (as if there was some internal bleeding) and then when I added some Stress Coat to the tank I noticed he had floated to the top and was stuck to the filter, I scooped him out with a net (thinking he was dead) but found he was still breathing (just) so floated him in a bowl at the top of the tank but within about 10 minutes he was also dead! I am really concerned about what is going on here as I can't find anything in my fish books that equates to what I've seen. As I don't want this to happen to my remaining fish I would really appreciate some help or advice! Thanks!
P.S. We have been checking the water twice a week and everything is as it should be (although KH is quite high in this area) and the temperature is about 25 degrees.

 

[dwarfgourami]

First of all, your water stats would help- ammonia, nitrites and nitrates.

 

[Cardinal]

Sorry, I'm at work just now but will check the water again when I finish and report back to you with the details. I have dip sticks that check NO2, NO3, KH, GH and PH but not sure how to check for ammonia!

 

[Cardinal]

The stats for my tank are NO3 = 25, NO2 = 0, GH = >16, KH = 6, PH = 7.6, temp = 25 degrees.

 

[Tokis-Phoenix]

I would definatly go and get a kit that tests for ammonia as this could be a very likely cause of death if the tank is new, fish have red/irretated gills and some are having swim bladder problems/not been able to stay upright. I would do a 40% water change right now with dechlorinator as if there are ammonia problems this is the best way to lower the levels of it and it will be good in general for your fish.

 

[Inchworm]

Hi Cardinal

I'm sorry to learn about your corys.

How often do you do water changes, and how much do you remove at a time? How long had you had the corys?

Whenever a cory shows any sign of illness or distress, the first thing to do is a big water change. I would replace 40% or more. It can often head off serious problems and is something you will want to do prior to medicating anyway.

By all means, get a test kit for your ammonia.

 

[Cardinal]

Thanks for the replies! I will go and get an ammonia test kit first thing tomorrow! We have been doing a 30% water change every week (more frequently when we first got the tank). We'd had both Corys for about 5 weeks.

 

[Cardinal]

I've now done a check for ammonia and it is 0.25 ppm (mg/L) (whatever that means), so according to the chart it is quite low although not as low as it should be, could this be what attributed to the death of my two cory's or do you think it might have been something else? Any advice would be appreciated so that I don't have anymore strange and sudden deaths!