And I agree with both: Don't add oxygen after about 24 hours into fermentation. You're courting wet cardboard/sherry oxidation flavors by doing so. Make a fresh starter, well-oxygenated, and pitch it while it's still fermenting - don't wait for the yeast to floculate out. Swampwater's question is a good one as well: How long has it been fermenting? You started above 1.1, that's a very big beer. If you aren't where you need to be after a week, no worries as long as it's still fermenting. But likely your yeast gave out in the anaerobic, acidic, alcoholic environment of that very big beer. Next time:
- A monster pitch, a two-liter starter at minimum.
- Extra oxygen BEFORE 24 hours into fermentation.
- If you can, use oxygen, not air, for oxygenation.
- Consider finishing the last third of the beer with a supplemental yeast. Champagne yeast works well, just be sure you hydrate it, or use a fresh starter of something like San Diego Super Yeast.