Last week I did a 20% water change on my 300 gal indoor tank that houses my pond fish. I had neglected to do a water change in two weeks and noticed by white fantail was spending alot of time on the bottom of the tank and the white butterfly koi (about 12" long two years old also biggest fish in the tank double the size of the goldfish housed with) had a red spot on his top fin. I added salt at 1 tsp per gal and 20 tsp of the Pimafix. (Directions say to use 5 ml per 38L of water). I didn't have more of the Pimafix. The next day the fantail was dead but the red spot on the koi had gone away. I worked two 12 hour shifts the next two days and didn't check the tank. When I checked my tank on the third day the koi was floating on the top of the tank dead alot of red streaking on its body. Checked ammonia levels and they were high .50ppm. I did an immediate water change, added aeration and 2tsp salt per gal. Some of the white and red goldfish white parts looked red at this time. By that evening the red had gone away.

I think the fantail was sick already but I'm wondering what killed the koi. As I hadn't checked tank for two days and the koi was obviously dead for longer than a day I'm thinking I had an ammonia spike or a severe depletion of oxygen. I'm running a Fluval XP filter on the tank and usually don't run the air stones as it gets alot of air going through that filter/pump. The ammonia levels are 0ppm with the Seachums' test (I use Prime as a water conditioner) after the water change.

I had used the Pimafix once before on a 10gal aquarium to treat a wakin that had a ugly red growth on the top of the back fin. Used salt as well but only 1/2tsp per gal and had aeration going and the fish was dead the next day after using the Pimafix. The lump on its fin had fallen off--don't know if the fish ate it or what.

I guess the bottom line of the story for me is to check water parameters more frequently and do regular water changes. I was going to have to sell the koi at the end of next summer as 300 gals is not big enough to overwinter koi but it was a shame to lose the biggest fish I had this way.