All snakes have different growth rates and your snake is growing just fine. My cali king was about 16 1/2 inches when I got him in May and now is 29 inches long pushing 30 and is in a shed cycle. The thing grew FAST! I think a 5 gallon is a little to small for the size of your snake BTW and you should move up to a 10 but a 20L would be better.
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Reptiles
0.1 - Early 2002 Anery Motley Corn 33 1/2"
1.0 - Late 2003 Yellow Rat Snake 15 2/3"
1.0 - 2003 Banded California King Snake 29 2/3"
1.0 Green Anole
0.0.0 House Gecko
0.1 Painted Turtle
Mammals
4 female ferrets,1 male cat and 1 male doberman...and a few kids!
RIP - Nala my Rottie 08/19/2004