What is the bracha for a fiber supplement that you mix in a glass of water? If in the morning I accidentally said the wrong bracha, e.g., al nidilat yadayim instead of Asher yazar, what should I do?

If it's a powder that you just mix into water: if it has a good taste, make "Shehakol", but if not, it depends if you enjoy the water (Shehakol) or if your'e just taking the water to swallow the supplement (if so, no bracha necessary). If you say a wrong bracha, then you should say immediately: "Baruch Shem K'vod Malchuto l'olam va'ed". And then you can and should say "Asher Yatzar" until you feel a new "need" for the bathroom.