I use a swamp cooler to keep my fermentation cool: a bucket full of water where my carboy or bucket goes in, and a t-shirt draped over the carboy/bucket with a fan pointed at it.
One room has two carboys in it and no sunlight hits them, so I use a white t-shirt over those and use bleach in the water of the swamp cooler.
However I recently just set up two carboys in another room where light now hits them, so I use a black t-shirt over those. At the time I set them up 4 and 5 days ago, I used some star san solution left over in the swamp cooler. But now that it has been over 3 days, the star san isn't sanitizing.
I can't use bleach in the swamp coolers with the black t-shirts as it will bleach them white and allow sunlight penetration.
Liquor is too expensive. Star San likewise.
What is an alternative that fits my needs?