Seems slightly unusual to get 8ppm ammonia readings this quickly.
Are the RCS still alive and out and about etc?
If it's really a ammonia issue I would have thought the RCS would either be going around the tank madly or dead.
I think RCS are great indictors of water parameters.
The new JBL Fertilizer, would that have caused any issues?
Particularly in making cloudy water and false readings?
And lastly, have you stirred or recently did a deep gravel vac on the substrate?
Aside from the comments already made above, that's all can think of at the moment.