Fatih,
Were these tanks where you're experiencing mobidity levels established with biofitration ahead of time? If not you may be experiencing "New Tank Syndrome." That is what Scott is driving at when he asked about ammonia, nitrite, and nitrate levels.
http://www.firsttankguide.net/newtanksyndrome.phpNitrate levels are not an issue, and the species you shipped are nitrite tolerant, but ammonia can be extremely toxic and even damage gill tissue. Even if your tanks are cycled, the ammonia and nitrite levels may have reach very stressful levels during transport. Did the shipping water have any salt in it or an ammonia nitrite neutralizer?
No doubt the fish were severely stressed by shipping even if your conditions are good now. It's a given you will lose some. If you're lucky the problems will subside and the survivers will be good strong future brood fish.
And yeah as said fathead minnows have short lifespans. They also get a lot of abuse by producers, suppliers, and bait dealers. A lot of bait store dealers don't have the first clue about biofiltration and practice none. They lose fish every day and consider it normal.
I bought fathead minnows from a local Amish baitstore a few weeks ago. He had them in well water temp water which was 51.6 F. He wasn't there as the bait purchase is an honor system, but I had to wonder how many anglers dropped them in 85 F. water right away which meant instant shock. I tempered the ones I bought for an hour before putting them in the pond.
Actually that cold of water was great for D.O. but it's a great way to get a secondary fungus infection going in warm water fish from handling.
Heard about a fish farmer that thought he was smart holding bluegills in cold water in the summer, as he could hold more in the tanks due to higher D.O., and feed them less. But many of them came down with fungal infections.
