Our 1/2 HP is the smallest size our manufacturer makes. I did find some 1/3 HP submersibles
at Grainger, although their 1/2 HP models can be bought for less (the 1/3 HP does only need 8 Amps at 120 VAC, though).
Extrapolating from the performance curves I have, george, a 1/5 HP pump (if found) should definitely go down to a 20 foot source, and I bet a 1/10 HP pump would as well.