My first one years ago (when I was a young teenager) was in a 1 gallon tank with a filter flow so strong, all he could do was sit in the corner behind the filter and sulk. Poor thing.
I started my 2nd one out in a half gallon tank not knowing much better, but fully intending to upgrade eventually. But she got sick and I upgraded much sooner to her 5 gallon tank, where she currently commands her territory quite well. My 3rd started out in a 3 gallon, but his 5 gallon is on the way, but it will be another few weeks when all the parts get here before he moves into it.