Good evening.

My question is mainly related to aquatic/semi-aquatic species. Say, marsh frogs.
It is known that, being kept in aquarium, frogs would eventually die (in 2-3 weeks max., I believe) if water is not replaced occasionally. Frogs are pooping, secreting some kind of skin excretions and turn their pool into something incongruous with living.
My question is, what are the most crucial properties of dirty water that actually lead frog to death. I mean, exact chemicals and concentrations, as well as their source in aquarium.
Well, as far as I know, ammonia and ammonium are the most damaging factors, deriving from excrement. Do you know any other chemicals that have chance to infest frog's aquarium in deadly amount? For example, are there any special skin excretions capable of dissolving and accumulating in water and becoming dangerous to frog's health at certain point. Are there any special poisons produced by aqua plants if one chooses to keep them in frog's tank?
And probably, there is something in addition to ammonium(ia) in excrements...
Also, I'm very interested in finding any numbers regarding the topic, like critical ammonia and other chems concentrations, recomended concentrations, concentration of this chemicals in natural pools/lakes the frogs live in etc. Please, let me know if you have ever found any data on this.

Thanks in advance.
Anton