I’m desperate for some advice. I’ve been keeping koi for about 3 years without any major issues, but over the last 48 hours I’ve lost two fish and a third is looking very lethargic. I’m worried I’m going to lose the whole pond.

The setup:

· Pond size: Approx. 4,000 litres (liner pond)

· Filtration: Oasis Clearwater 4-in-1 pressure filter with UVC

· Pump: Oasis 6,000 lph

· Stock: 5 koi (largest around 35cm), 3 goldfish

The symptoms:

· The fish that died were hanging near the surface, fins clamped, and appeared to be gasping.

· One had some red streaking visible in the tail fin.

· The remaining fish are sitting on the bottom. One is flashing (rubbing against the bottom) occasionally.

· No visible white spot, ulcers, or fungus that I can see.

Water parameters (tested just now with API Freshwater Master liquid kit):

· Ammonia: 0.25 ppm (this is worrying me)

· Nitrite: 0 ppm

· Nitrate: 20 ppm

· pH: 8.0 (normal for my tap water)

· Temperature: 12°C

What I’ve done so far:

· Did a 20% water change yesterday with NT Labs Tap Water Safe dechlorinator.

· Cleaned the filter (in pond water) but didn’t touch the media too much.

· Added an extra air stone.

The only thing that changed recently was that I fed them a few pellets last week when we had that warm day. I’ve stopped feeding now.

I’m seeing ammonia even though I haven’t fed in days. Is this a filter crash? Should I do a massive water change or will that make things worse? I have some Blagdon Pond Salt and NT Labs Anti-Parasite treatment in the shed, should I add anything?

Any help would be massively appreciated. I feel sick watching them like this.

Thanks.