Half of my fish suddenly died due to a believed nitrite spike and I have no clue why? My 48 gal bowfront has been running for 7 years now with a variety of fish and a well developed biological system. I did a 50% water change around 4pm and tank seemed to have no issues. 8pm tank seems fine. 11pm, half the fish are dead. Other half transported to 55 gallon and survive. 125 gallon and 55 gallon tanks are fine and normal after their wc

Possible causes?

Temperature change- this was the first tank i did a wc for today and did not let the hose run before hand. The florida sun has caused the hose water to be warmer. Tank increased around 2 F.

Water Contaminant- Since I did not run the hose before, is it possible something was in the stagnant water?

Nitrite spike- i have never seen nitrites in this tank before so I am extremely confused as to what caused this spike.

Any suggestions or thoughts are extremely welcomed