Butterfly Koi Lethargic After New Koi Added

10,000-gal pond, 7-yr butterfly koi (28"), 1 other koi, many goldfish. Added 3 koi → butterfly happy, played. Now: bottom-dwelling, brief morning activity, eats little, tail reddish/orange hues. Others fine. pH/filter good.

Sequence Suggests Pathogen Introduction

New koi likely brought bacteria/parasites. Red fin streaks = bacterial infection (Aeromonas) or parasite damage.

Steps

1. Test ammonia → 0.
2. Watch for surface gasping (even with O₂).
3. Add mid-depth aerator (not bottom).
4. Consider quarantine + scrape of butterfly koi.
5. 10,000 gal = hard to treat; prevention > cure.

New fish quarantine (4–6 weeks) prevents this. Damage may be done.