Fish stores keep there levels low to keep disease and parasite activity low and stimulate feeding.
If you have anything in your tank other than fish 1.018 will not work. Ph will be harder to maintain as well as all other essential elements. Salt mixes are designed to be mixed at around 1.025 so by lowering the salinity you are also lowering everything else, CA
etc. If you have corals and inverts you should maintain levels between 1.024- 1.026 to keep them happy. Be sure you slowly acclimate fish to your salinity. 1.018 to 1.025 is a strressful change for fish if not done over days.