"Water Chemistry and the Nitrogen Cycle"
If it's up for a vote, I'd pick this one. SO important, simple, yet still not understood by many.
It's not up for vote, but I'll take your opinion anyway.
Too wide a field. A whole university fisheries department couldn't properly deal with your list.
Narrow it to one question that can be researched, answered, and defended.
The list is possible topics, I'm not going to do all three.
Maybe... "the importance of the nitrogen cycle in a freshwater aquarium"?
Okay, I'll look into it. Maybe as part of the Water Chem and Nitrogen Cycle idea.
cycling a tank really isnt mimicking nature. we put fish that would normally have a huge rang in the wild into a little box.
most of the time when people talk about mimicking nature its looks which most fail to do.
most of the so called natural looking tanks really are not that they are made to look nice very very few actually looks like something you would find the in the wild.
Cycling a tank is mimicking the natural nitrogen cycle. There is a global nitrogen cycle that keeps us from wallowing in ammonia and nitrite
Ah, so my planted tank doesn't attempt to look natural then? I try to make my tanks as natural looking as possible, because I've always liked nature.
Last I knew there was a "natural" N cycle pretty much like in our aquarium...sure, it's a lot more complex but the basic processes are the same.
The Nitrogen Cycle is not limited to a fish tank. Every body of water has a nitrogen cycle, as does the entire planet. All that ammonia has to go somewhere...

Are you supposed to be using actual scientific, peer-reviewed sources? If so, your school should subscribe to a number of databases in which you can search for articles relating to your topic.
Yes, I need scientific peer-reviewed sources. I have a few now, but the more the merrier. Also, the school does have a number of databases, but we're also supposed to get most of this done on our own.