Sick Fish?

[yellowjacket171]

2 months ago I bought 2 kuhliis, 4 various corys and 5 female platys. A week later a little kuhlii died. The other has been surviving and is getting big. About a month ago I started to lose corys. My 2 spotted corys died within a week of each other. They were big when I got them so I thought maybe it was age. I thought the second spotted cory died of loneliness because they only hung aroung each other. 2 days ago I lost an albino cory I had for 1 1/2 years and early last evening I found my peppered cory dead which I also had for 1 1/2 years. I was gone for the day and found him dead when I got home. After a death in my tank I always test my water and the tests come out with excellent results. I also do a cleaning and a water change. I take great care of my tank doing weekly water changes and partial cleanings every 2 weeks. I also examine the dead fish under a magnifying glass to see if I can see anything wrong with them. The one fish that died last evening had a hole in his underside near the chest area. It was kind of yellowish around the hole. Do you think the others might have picked at his body after he was dead and that is why there is a hole in his body? I always examine my fish everyday when I feed them and do a head count. Everyone looks healthy and is eating well before they have died.
I have 2, 3 or 4 week old baby platy fish that have been surviving in my tank and look very healthy, always eating. I have not seperated them from the others because there is plenty of hiding spots for them. You see I had gotten the 5 female platys because I didn't want babies. 2 days after getting them I noticed one was pregnant and noticed the babies about 3 or 4 weeks ago. The only meds I have put in my tank is Melafix.
I want to get a couple more corys and another kuhlii but not until I know if anything is affecting my corys. Any advice will be helpful.
Cindy

 

[Menagerie]

You see I had gotten the 5 female platys because I didn't want babies. 2 days after getting them I noticed one was pregnant and noticed the babies about 3 or 4 weeks ago.

Female livebearers store sperm and can have fry with no male in sight. Males are the only ones that will not have fry

The one fish that died last evening had a hole in his underside near the chest area.

It could be an internal parasite or worms.
Check out these links to see if something looks familar:
http://www.aquatronicsonline.com/hobbyist/hobbyist3.htm
http://fish.mongabay.com/diseases.htm
http://www.fishpalace.org/Disease.html

 

[MarkLehr]

Could be internal parasite. Could be your fish picked at him. Fish often look this way after being dead in a tank for several hours & being picked at.
Your Kuhli probably was just not healthy to begin with or did not transport well. They are known for having difficulty acclimating at the pet shops.
Your spotted Cory did not die from loneliness. I suspect you may have a bacterial infection, because Corys tend to be very prone to this and Platy's do not. I hate adding medication to the water. I would suggest you first try a series of frequent water changes, say 10 gallons in the morning & 10 at night, for 4 or 5 consecutive days. If that does not help, then consider medicating. IF YOU DECIDE to medicate, do not use a copper based medication.

 

[MarkLehr]

Oh yes, one more idea. Cory's are very susceptable to pH swings. Have you tested this?