Common Newbie Pitfalls

[BrianNY]

I thought I would make this a sticky for awhile and gather some thoughts for discussion on the subject. In this way anyone contemplating setting up a tank for the first time can read the thread and be aware of what to look for.

Most people starting an aquarium think... Add water, add fish, and that's all there is to it. Taking on just one topic at a time (eg. N cycle), please add some common pitfalls that you can think of.