A tank top? I associate that with something woollen from the 1970s or particularly tacky from the mid-80s. Must be a new fashion or transatlantic difference.
Just wear what you want and what you're comfortable with. Shorts and t-shirts in the day time, flip-flops if you must, though sandals are probably more comfortable if you're expecting to walk a lot. Perhaps a shirt and trousers at night if you're dining somewhere nice, though most of the city is very informal.
It will be hot and humid.
Here are some photos from last August that should help you realise that there's not really a look to aim for. If you're under 30 then you could start cultivating the local 'gypsy' style now which might help you fit in but realistically you'll stand out a mile. We visit the area often, wear clothes that aren't disimilar to some local styles and one of us speaks very good Spanish and even a little Catalan - but everyone still knows we're tourists. Without meaning anything offensive or patronising we're pretty good at spotting North Americans, too.
For fitting in at a castellers session, white cotton trousers, a faixa, several mocadors and uni-coloured long-sleeved shirt will work, but expect to get roped in to the pinya, which will take you to new levels of heat and humidity.
For a correfoc, canvas jacket, a hat the covers the hair and some mocadors for the smoke and to give yourself a more arch and sinister appearance. Boots can be handy, too.
The pics might also give you some ideas that haven't occured to you about what do in in Barcelona in mid-August.
Edited: 9:02 am, July 19, 2013