Understanding how the preservation process affects fish morphology is important for studies that use museum collections as voucher specimens. Sixty-nine Rio Grande silvery minnows (standard length [SL] 32.14–81.65 mm) were observed over 545 days during a formalin-to-water-to-ethanol preservation procedure. Median standard length decreased by the end of each preservation step; specimens shrank 1.06 mm in formalin solution, 1.97 mm in 35% ethanol, 2.17 mm in 50% ethanol, and 2.48 mm in 70% ethanol. Peak shrinkage occurred at 365 d, with a median decrease of 3.86 mm (SL 29.57–75.98 mm). After 545 days, Rio Grande silvery minnows began to increase in length, exhibiting a median shrinkage of 2.04 mm from live length. Research on museum specimens that includes morphological measurements should consider that changes in length or body shape may influence or hinder the ability to detect changes in morphology over time.