The aim of the study was to analyze the content of Ni in fractions of 1.0–0.2 mm, 0.2–0.1 mm, 0.1–0.063 mm, 0.063–0.02 mm, and <0.02 mm of river bottom sediments and plants (root, stem, leaf) of broadleaf cattail (Typha latifolia), yellow water lilies (Nuphar lutea), and cowbane (Cicuta virosa). Nickel content in bottom sediment was slightly higher than the geochemical background. The largest amounts of Ni were associated with the finest fraction, while the smallest with the thickest fraction. The highest content of this element was recorded at the point Ełk Barany, which was connected with developing industry in the city of Ełk. All of the tested plants showed greater Ni contents than the amount naturally occurring in plants. In most cases, most of Ni was associated with plant roots, and the least with leaves. It has been proven that aquatic plants can be good indicators of the aquatic environment status.