Koi lying on side... dying

[gsparsan2]

Hi All, I need some urgent assistance. I have a 6000L above ground pond with 7 koi's, 2 IR Sharks and some wild guppies. I've had these fish for over 7 years. This morning feeding time the behavior of the Koi was very strange. Normally at this time they are hyper active, parading in front of the glass panel in front, waiting for food. Today the sharks were doing the same but ALL the koi were 'resting' down the bottom of the pond. When I watched closer, some of them were rolling on their sides or going vertical, resting on the wall of the pond. I have never seen that before. As at last night they were all looking fine.

I came back after 1 hour and all of them were on their sides, hyperventilating. The behavior of the sharks and the guppies are completely normal. I am panicking and really don't know why and what should I do. I am following the advice I have myself given others many times. In case of doubt, water change. So I am proceeding with a 70% water change.

Please if you have any advice or have seen this before, please tell me what to do.

 

[gsparsan2]

Just an update. I have treated with Oase Universal for mixed infections, under recommendation from my local pet shop. I don't think it has done much and I still don't know the cause of the problem. 7 days on, I have lost 5 out of 7 Koi's. The 2 remaining seem to be getting better. They have started eating again and are swimming around. Still sluggish but at least not lying down on the side like before.
One of my IR sharks seem a bit slower than usual but feeding normally. Let's see.