It's a good idea to let a bit of time pass after washing your hands before touching aquarium water, but the exact timing depends on how you've washed and rinsed. The main concern is leftover soap, lotion, or sanitizer residue, which can harm sensitive fish or disrupt the aquarium’s ecosystem.
After washing your hands, rinse thoroughly under running water for at least 30 seconds, focusing especially on your fingertips and under your nails. Then, dry your hands with a clean towel. It's wise to allow your hands to air out for about 5–10 minutes after drying. During this period, your skin is more likely to shed or evaporate any lingering traces of soap, especially when fully dry.
If you used heavily scented or moisturizing soaps, use extra caution and consider a longer wait, or rinse again with plain water before air-drying the second time. Some aquarists even keep a separate, unscented bar soap (like Castile) just for tank maintenance purposes.
Overall, the goal is to ensure your hands are as free of residues as possible. When you’re sure your skin feels clean, dry, and scent-free, you can safely work in your aquarium. Better safe than sorry, especially with sensitive fish.