Mathematica is not good at creating beautiful and small gifs. It won't reach the quality you can can with e.g. ImageMagick's convert. Therefore, I suggest you export your images as png

Table[Export[
  "~/tmp/imgs/" <> 
   ToString[PaddedForm[i, 3, NumberPadding -> {"0", "0"}]] <> ".png", 
  tt[[i]]], {i, Length[tt]}]

and then you call something along the lines

convert -delay 1x15 imgs/* +dither -layers optimize out.gif

in a terminal

[![enter image description here][1]][1]

I have to admit, that for this particular example, the gif created by Mathematica does look good as well. The file size, however, is larger by almost a factor of 2. [1]: https://i.sstatic.net/FOSK8.gif

Mathematica is not good at creating beautiful and small gifs. It won't reach the quality you can can with e.g. ImageMagick's convert. Therefore, I suggest you export your images as png

Table[Export[
  "~/tmp/imgs/" <> 
   ToString[PaddedForm[i, 3, NumberPadding -> {"0", "0"}]] <> ".png", 
  tt[[i]]], {i, Length[tt]}]

and then you call something along the lines

convert -delay 1x15 imgs/* +dither -layers optimize out.gif

in a terminal

I have to admit, that for this particular example, the gif created by Mathematica does look good as well. The file size, however, is larger by almost a factor of 2.