i would probly just use a 20g long. but depending on how much high you can go you might be able to get away with a 30g breeder(13" h) or a 40g breeder(17"h)
here is a list of different sizes i used from a long time ago
i stopped at 24" cuz any smaller wont do you any good imo)
(L x W x H)
15 Gallon 24 1/4 x 12 1/2 x 12 3/4
20 High 24 1/4 x 12 1/2 x 16 3/4
25 Gallon 24 1/4 x 12 1/2 x 20 3/4
30 X-High 24 1/4 x 12 1/2 x 24 3/4
20 Long 30 1/4 x 12 1/2 x 12 3/4
29 Gallon 30 1/4 x 12 1/2 x 18 3/4
37 Gallon 30 1/4 x 12 1/2 x 223/4
26 Flatback 36 1/4 x 12 1/2 x 16 5/8
23 Long 36 1/4 x 12 5/8 x 13
30 Gallon 36 1/4 x 12 5/8 x 16 3/4
38 Gallon 36 1/4 x 12 5/8 x 19 3/4
45 Gallon 36 1/4 x 12 5/8 x 23 3/4
30 Breeder 36 3/16 x 18 1/4 x 12 15/16
40 Breeder 36 3/16 x 18 1/4 x 16 15/16
50 Gallon 36 7/8 x 19 x 19 5/8
65 Gallon 36 7/8 x 19 x 24 5/8
HTH