davidfisher189 said · 1 Apr 2026, 02:21
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
Hi there — first off, I’m really sorry you’re going through this. It’s clear how much you care about your fish, and it’s absolutely gut-wrenching when things start going wrong after years of no trouble. You’re doing the right thing by stepping back, testing, and asking for help.
Let’s go through this step by step. You’ve given really useful information, which makes it much easier to help.
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1. The ammonia reading is key
You’re right to be worried about that 0.25 ppm ammonia. Even at low levels, it’s toxic — and at 12°C, your filter bacteria are working much more slowly than in warmer months. The fact you’re seeing ammonia despite not feeding suggests something has disrupted the biological filter.
Given you cleaned the filter recently (even carefully), combined with the temperature drop and possibly a small ammonia spike from the pellet feeding last week, it does sound like the filter is struggling to keep up — a partial “filter crash” or mini-cycle.
The red streaking and gasping at the surface are classic signs of ammonia stress. Flashing can also be irritation from ammonia or water quality issues, not necessarily parasites.
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2. What to do right now
Don’t do a massive water change all at once — that can shock them further. Instead:
· Do another 20–25% water change today, dechlorinated and temperature-matched as closely as you can.
· Repeat daily until ammonia stays at zero. With 0.25 ppm showing, small daily water changes are safer than one huge one.
· Keep the extra air stone running — that’s excellent. Low temperature holds more oxygen, but the gasping suggests they need all the help they can get.
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3. Should you add salt or anti-parasite treatment?
Hold off on both for now.
· Pond salt can help with nitrite toxicity and reduce osmotic stress, but it won’t fix ammonia directly, and once added it’s difficult to remove. If you do use it, use it sparingly and only once ammonia is under control.
· Anti-parasite treatments can sometimes make things worse if the root cause is water quality. They can also knock back the filter bacteria further. Right now, the symptoms point much more strongly to ammonia/water quality than parasites.
Once the ammonia is consistently at zero and fish are still showing flashing or redness, then consider a treatment — but water quality first, always.
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4. Filter management
You did the right thing cleaning the filter in pond water. At this point, avoid cleaning it again until the ammonia is stable. The bacteria are struggling already. If the UVC bulb is older than 6–12 months, it may be less effective at controlling water clarity, but that’s not urgent right now — focus on ammonia.
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5. Keep feeding stopped
You’re spot on there. At 12°C, koi can’t digest food properly anyway, and any uneaten food or waste will add to the ammonia. They’ll be fine without food for a week or more while things settle.
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6. What to watch for
Keep testing ammonia daily. If it rises above 0.5 ppm, do an extra water change. If nitrite starts to appear after ammonia drops, that’s actually a sign the filter is recovering — just keep up water changes to keep nitrite low.
You’re not powerless here. The fact you’ve caught this and are acting now gives them a real chance. I know it feels awful watching them, but small, steady actions over the next few days are the way through this.
You’ve got a good setup, you’ve been keeping them successfully for three years — this is a rough patch, not a failure. Hang in there, and let us know how they’re doing.
