The main active ingredient in detergents – whether that's your hand soap, laundry detergent or those you use in your kitchen – are surfactants. The job of these chemicals is to reduce the surface tension of water so that the emulsion of water / dirt can be carried away. The way this works is by the surfactant molecules arranging spheres around the target stain to lift it away in the water. This works perfectly for us. So far, so good.
The problem is that an awful lot of this can end up in the seas and oceans, and the very types of substances we want to remove from our hands and clothes have similar properties (as far as surfactants are concerned) to the mucus layers used by fish to protect them from parasites and bacteria. It's basically all the fish has to protect itself from from all the threats that exist in the water in which it lives. Surfactant concentrations as low as 2ppm (1ml in 500 litres) will double the amount of chemicals it will absorb from its environment.
It's not new information. There are studies from nearly 50 years ago which outline this for us. Obviously we can't go back to a world before hand washing and general improvements in hygiene, but the very least we can do is use biodegradable soaps and detergents, which will at least decompose in the environment.