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Hydrotherapy is great! Your muscles are flushing out all of the waste that has been lurking around, your blood flow increases. You are also not sitting there snacking, like you might be doing in front of the TV.
You got to drink pleanty of water to rehydrate yourself!
Your blood flow probably does increase, and I bet you are sweating some that could account for some loss, but if you are habitually good at staying hydrated, I doubt that could account for 5-6 lbs of persistent wt loss. How much does your weight fluctuate normally, and how often do you weigh in? Perhaps you caught one fluke high and one fluke low reading?