In answer to your question yes, Osmocote DIY root tabs can add ammonia if the right one isn't used. Also check your ph because if it's under 6 your biological filtration stops and begins to die off causing ammonia. My bet is the root tabs. How many did you use?
You need to continue doing WC
and if possible can you use a gravel vac to suction out the root tabs? Just a thought. You need to get the ammonia down to .25ppm or you will most like continue to lose fish. Also use Prime or Amquel (I think that is the name) to convert the ammonia into a less toxic form. Follow the directions on the bottle as to how much you need to use and how often.