Surprise Phone Call... All the fish are dead

[THX1138CY]

Have you considered getting 8 bottles of the 250ml IO BIO-Spira, and use that to convert all the NH4 to NO3/N2, when the NH4 reads zero start adding vodka/vinegar, so the bacteria can convert the NO3 to N2, this will give you enough bacterial population to prevent this type of crash from happening again. Crashes suck, but you can at least use the current conditions as fuel for recovery and prevention.