Energy Recovery (ERII) had the highest return in October 2015 by a small-cap US stock, returning 231.9%.

ASSET	MONTH	% RETURN
Energy Recovery (ERII)	October 2015	231.94%
LifeMD (LFMD)	October 2015	160%
Obsidian Energy (OBE)	October 2015	143.75%
Anavex Life Sciences (AVXL)	October 2015	88.93%
Canopy Growth (CGC)	October 2015	64.42%
Lincoln Educational Services (LINC)	October 2015	56.67%
InnovAge (INNV)	October 2015	54.17%
ANGI Homeservices (ANGI)	October 2015	53.37%
Pacific Biosciences of California (PACB)	October 2015	51.06%
Idaho Strategic Resources (IDR)	October 2015	50%
Rayonier Advanced Materials (RYAM)	October 2015	49.19%
Empresa Distribuidora y Comercializadora Norte SA ADR (EDN)	October 2015	43.1%
World Acceptance (WRLD)	October 2015	42.6%
Know Labs (KNW)	October 2015	41.51%
Northern Dynasty Minerals (NAK)	October 2015	41.48%
NETGEAR (NTGR)	October 2015	41.39%
Tronox (TROX)	October 2015	40.82%
Seabridge Gold (SA)	October 2015	39.66%
Genie Energy (GNE)	October 2015	38.97%
Gibraltar Industries (ROCK)	October 2015	38.06%
Arcturus Therapeutics (ARCT)	October 2015	37.46%
Oil-Dri Corporation Of America (ODC)	October 2015	36.2%
Heidrick & Struggles International (HSII)	October 2015	35.93%
Theravance Biopharma (TBPH)	October 2015	35.29%
Insteel Industries (IIIN)	October 2015	33.77%

Small-cap stocks are treated as having a market cap between $300M and $2B.