dead fish sinking

Question
55 gallon aquarium...had for over two years...Rena Filstar XP3 canister filter.  change water once every two to three weeks.  Usually keep 3 or 4 baby koi at a time.  When they get to be about 4" long than I either sell or give them away.  Water tested perfect for nitrate, nitrite, hardness, alkalinity and ph.  All three fish died in one night and they were laying on the bottom of the tank like they were asleep.  Not a mark on them...no sign of ick or parasites.  I have never seen a dead fish on the bottom of the tank before and when I got them out with the net they seemed to be waterlogged.  Tank is empty of fish until I figure out what happened.  Thank you for any help you can provide...the employees at the pet store were puzzled.

Answer
Hi Sheree;

Often when fish sit at the bottom like that it's from nitrite poisoning, but your tests showed nothing. Weird. It's possible they had the parasite ick, but only in their gills. Or, another gill disease the has caused deterioration of the gill filaments and then they died from suffocation.

Here is a link to info about gill diease;

http://www.thetropicaltank.co.uk/hdgilld.htm

At Your Service;
Chris Robbins