It wouldn’t be strong enough to cause any harm to a stream or river, by the time it’s hit the ground it’s nutrilised. Not to mention the fact the average house softwash is a drop in the ocean when compared to a stream/river etc.
My dad did hear of someone who killed a load of fish in a local large pond, he disgaurded a huge volume of hypo not realising it went straight into the pond.
I’ve no idea how it happened!
My old man dealt with hypo all day long as a technical manager of a local leisure Center. They used hypo for the swimming pools and leisure pools. He’s had it in his eyes and all sorts!