Cath Palug's revelation made me think about what kind of poison the dragon was producing for it to contaminate the lake, but then it turns out, the dragon is not intentionally trying to poison the lake, it was trying to wash off the poison by submerging into the lake. The problem is that the poison has now mixed with the water and turned the water into lethal poison. And since this is a Primordial Poison, it's going to be impossible to remove, or if it is not, it's going to be a big work to do.
If that is the case, this will be a really hard job for all of us. But I have a plan to do and although this will be the first time I will be doing this, I needed to employ everyone's help. But first, we will have to remove the dragon from the water and ensure this particular dragon either get the help it needs or if possible, mercy kill it.