Every day, we are acquiring more and more clues regarding the effects of different spectral structures (SS) of light on ocular axial length (OAL). As a step towards understanding this association, this study sought to characterise the effects of light sources of different SS on OAL in fish through comparisons with indoor daylight. The experimental design was completely randomised with 4 treatments and 2 replications. Three hundred and fifty two rainbow trout (Oncorhynchus mykiss) were housed in 8 tanks and fed for 71 days. Differences in the mean values of ocular elongation were determined at the end of the experiment. The daylight group was exposed to indoor daylight in the hatchery environment, the red group was exposed to long wavelength light (600–650 nm), the green group was exposed to mid-wavelength light (495–570 nm) and the blue group was exposed to short wavelength light (420–495 nm). The values of the OALs in fish grown under the same light intensity, but with light of different spectral characteristics, demonstrated significant differences (p < 0.05). The mean OAL in the daylight group was determined as 3.64 ± 0.40 mm, as 3.70 ± 0.35 mm in the red group, as 3.53 ± 0.34 mm in the green group and as 3.42 ± 0.29 mm in the blue group. The mean OAL in the blue group was significantly shorter compared to the red (p = 0.003) and the daylight groups (p = 0.02). When compared with the long wavelength light and indoor daylight, the effect of short wavelength light on OAL in fish was observed to be negative. Exposure to light with modified SS of in indoor environments may be effective in stopping ocular elongation.