Koi suddenly died

[Sushi Prohibited]

Using liquid test kits from Pondcare and Tetratest:
PH 8.6
Nitrate NO2 0ppm
Phosphate PO4 2.0ppm
Ammonia NH3/NH4 0 (another test kit said 1.5)
GH never changed the water color red to green after 20 drops
KH 9 drops

 

[WaterGardener]

Do a search here for algaecide killing fish. It's not at all uncommon. You are fortunate that only one fish was lost.

Please stop adding those products to your pond and give it time to balance itself. It has gone through some major changes lately and needs time, lots of time, and your patience to regroup.

 

[Sushi Prohibited]

Thanks everyone!