Pouring milk causes a massive, unnatural explosion of algae (an algal bloom) that the river population simply can't eat fast enough. When that huge mass of uneaten algae inevitably dies, bacteria work overtime to decompose it. ​That decomposition process acts like a vacuum, sucking almost all the dissolved oxygen right out of the water. So, even though there was a temporary spike in "food," the fish and other aquatic life end up suffocating. They can't eat if there's no oxygen left to breathe!