Half my fish died

[dmk164]

im breeding fish but for some reason half of one species died i checked nitrates 10 ppm i checked ammonia not zero but maybe half of .25 ppm i think i crashed my cycle because ive been moving bacteria around should i check my nitrites ?
its a 40 breeder and had 25 2-25 inch fish in at tops
what should i do other than 50% water change

 

[Pwilly07]

I'm really sorry this happened. Please fill out the emergency template.

 

[jtjgg]

should i check my nitrites ?

yes, any time any thing happens check all the parameters.

please fill out the emergency template with as much details as possible.