Possible Contamination

Question
Hi i currently have a largish tank approx 80-100 ltrs., I have had 8 fish in it but two have died within a day and a half and i cant seem to find an explanation. The fish i have are: 3 common goldfish, 1 black moore, 1 red and white commet, 2 Shubunkin's, 1 english koi. The Koi and the 3 goldfish are new additions, ive had them in about three weeks now. One of the goldfish was looking abit poor and swimming close to the top of the water (slowely) for a day, then started to drift on its side. it was removed from the tank and then quarantined but died a few hours later. The next day i noticed my black moore's tail was a bit thin, then that died overnight. Ive since found out that fancy fish should not really go with other goldfish as they like to go for wavy tails. Possible cause of death?
I can see that one of the goldfish has a slightly misty eye, and some of the fish are now tending to sit on the bottom of the tank motionless. When feeding they all perk up and come to the top for food. Ive done a half water change with tap safe. Can anyone help or point me in the right direction? In help will be greatly appreciated. Thanks, Chris  

Answer
Hi Chris
I just say your question in our question pool, so sorry for the delay in answering.

Well, you're tank is roughly about 20-25 US gallons, and you shouldn't have any more then 2 maximum goldfish in there.  For goldfish it's usually recommended to have 15 gallons per goldie.  10 would be ok for one if there's good filtration and you do frequent water changes.  So, in a nutshell, your tank is way overstocked.  What's happening, is the ammonia, nitrites, and nitrates are going out of control.  Goldfish put out a lot of ammonia naturally through their waste and respiration.  Ammonia and nitrites in any amount is toxic to the fish, and that's why they're just hanging at the bottom and top of the tanks.  It's slowly killing them.  That's most likely what killed the ones that have died already.  
The one that had the thin tail, and the one with the eye problem, that's most likely caused by poor water quality.  With that many in that size tank, there's really no way to keep the water quality good.

Koi should only be kept in ponds.  They get huge, too huge for most aquariums.  Common and comets get anywhere from 8-12+ inches, and I'm not sure on the fancier ones, I believe they get around 6-8 inches.  

Sorry to say, but I would take back the koi and the 3 newer fish you bought.  As for the remaining fish, you'll really need a larger tank to keep them alive.  I would say go for at least a 200+ Liter tank.  Cut back on their feedings for now, at least to every other day.  They'll be fine, plus that'll help cut down on the extra ammonia being added into the tank.  You're going to need to do daily water changes, I'd do about 50% of the water.  

Hope that helps, and good luck!  Let me know if you have more questions!!

Christy