I am always very cautious when using soap when washing hands before I clean my aquarium. Soap is of course toxic to the fish and ecosystem and a small amount can do some damage. I usually do my work in the morning so I rarely have to use soap to get anything off my hands but I do rinse them well under warm water just in case. If you do have something on your hands that requires soap to really get off, use soap and then wait a while before rinsing again and doing work. Also, like Metalman said, skincare and moisturizers are very common. If you do have a skincare routine and or use anything on your hands and arms like a moisturizer, try to stay away from using them before cleaning your tank. However, this of course is all my opinion. Some people are much less worrisome about it, some much more. It's really up to how comfortable you are with the possible problems that might arise because of these things. I just like to keep in mind that anything that you wouldn't feel comfortable putting in your aquarium, shouldn't be on your hands before cleaning (Soap, Hand Sanitizer, Oils and Grease, Chemicals, Moisturizer, Skincare Products, Etc.)