If you have made a common beginner mistake in your fish keeping time, post it here. Hopefully, if beginners and newbies read this first, they will not make some of the same mistakes we have made. The worst mistake I made was when I first got my 10g tank. I just put the decoraions in it, filled it with water, and the next day, added about 10 fish. This was bad because I did not cycle the tank(check out this article if you are not sure what cycling is. Cycling). I also added way to many fish at once. When doing this, certain levels in your tank, like ammonia and ph can rise, killing all the fish. The most you should add to a tank at once is about 2-3 fish, unless you are adding schooling fish, then 5-6 is ok. Needless to say, within 2 days, all the fish were dead. Here is my site, check out the Getting Started page if you need some help or don't know what to do.