I would point out that gaseous ammonia is actually on the list of toxic substances as well. It's not just ammonia in an aquatic environment. Indeed, the reason that gaseous ammonia was put on the TSL is that it's a smog precursor. What underlies that is in fact that one of the major sources of smog, the third major source, is what are referred to as area sources--as opposed to smoke stacks and automobile tail pipes--which are essentially the volatilization of various substances from cleaners, paints, and other applications in both household and industrial applications. In fact, in a sense, cumulatively, the effect of ammonia in cleaners and those kinds of things is a potential contributor to smog. It also presents occupational hazards, and that's well documented as well.
So I'm actually surprised that one would point to ammonia as problematic in this context. I think it actually fits the bill rather tidily on a number of different fronts.