Absolute beginner here. Day eight of trying to coax flour and water into a viable sourdough starter. I rigged a proofing box out of a styrofoam cooler and a light bulb, and it's keeping the jars between 74-80 degrees. I started with a mix of 50/50 rye flour and AP flour, feeding twice daily with AP flour. Using a 1-1-1 starter, flour, and water ratio. There was a lot of rising activity on days two and five, hooch appeared on days three, four and five. Last few days no rise and fall and only small bubbles. A very thick skin forms on the top - like you get when you make pudding, only denser. Underneath that skin the slurry is puffy and light, though it does not rise. It smells like fresh flour with a whiff of ferment. It tastes very tangy/vinegary/sharp and feels almost fizzy on the tongue. Is that skin on the top a problem or a natural phase? What could be causing it? Any advice on how to manage it? Thanks very much.

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