Hi, i have never had a fish with costia, but i would have thought the symptms would the same as other parasites, sitting on the bottom, clamped fins, flicking etc. Im not sure what is used to treat costia but it may be worth taking one of your fish for a scrape, or doing one yourself if you have a microscope. Then you'll be able to see whether you have any other parasites and treat accordingly. With the koi dieing first being a doitsu it would suggest it is parasites as these are often affected first. Also, i dont personally quarantine new fish as i dont have the room, and i have a trusted dealer i have been going to for a few years now. But if you do, i would do so for longer than a week. I think KHV needs a fish to be exposed to about 27C for 2 weeks to show itself. And by raising the temperature over a longer period the fish will be more active and you might be able to spot any flicking etc earlier before introducing them to the pond. Hope this helps.
Posted on Aug 30, 2006, 3:37 PM from IP address 82.38.197.142