Tilapia can handle very high ammonia levels IF it builds up slowly, It rarely builds up slowly in RAS systems however!
Ammonia poisoning will cause blotches on the top of the head and make the fish look like it has a saddle on it. The fish will spin and swim like crazy in all directions and angles. They will also jump high out the water. Ammonia will also dissolve the gills, tail and dorsal fins.
