Yikes! Ammonia Spike!

Did my weekly water test just now and here are the results:

pH: 8.0
Ammonia: 0.50 ppm
Nitrite: 0.0 ppm
Nitrate 40 ppm

I haven't seen any ammonia reading since the cycle finished, so this is a bit of a shock as I haven't done anything out of the ordinary.

Going to do a water change here in a bit, but what could have caused this ammonia spike?

I had a fish die Friday for no apparent reason. I replaced the fish the same day, and added 15 ghost shrimp, which several of them died and got stuck to the filter, then were promptly removed. Could that have contributed to this spike?

I'll let you respond while I'm doing the water change and check back I'm done.

 

My guess would be adding more fish in would of caused a sudden spike.

 

Okay did the water change and cleaned out one of my filter boxes with a toothbrush to get rid of some algae and scum buildup. Removed about 20 gallons and added 24 to fill it up, also added a carbon packet. Add Stability x 2 in case I hurt any of my BB when cleaning.

 

how long has the tank been running?

 

Its been running for 3 months, completely cycled for 2 months.