I brewed an oatmeal stout about 6 weeks ago (recipe from the brewmaster's bible) partial mash with 8 oz. roasted barley and 6 oz. chocolate malt to 8 oz. rolled oats, 3 lbs. British pale, and a few lbs DME. Water was plain old supermarket spring water. Grains steeped 90 minutes at 150 F and boiled 60. Very little hopping. I used a London Ale yeast in some sort of new packaging that claims to be superior to smack packs by culturing the yeast right in the retail package. OG of 1.058, FG of 1.013. Kept fermentation in the low to mid 60s to limit esters. Primed with corn sugar and bottled. Everything pretty uneventful until I tasted it 3 weeks after bottling. It's not BAD but it's noticeably tart. Not getting worse either and does not seem to be the result of infection. It also has weak foam and relatively low-moderate carbonation. Is it the yeast, the water, or something else? Relatively new to brewing, crossing over from winemaking, and advice appreciated.