Know that little packet of additives that come with arranged flower bouquets? Apparently, it's just a little sugar, chlorine, and some citric acid. So, create your own DIY solution that you can continue to use throughout multiple water changes. "Mix 1 teaspoon sugar, 1 teaspoon bleach, and 2 tablespoons lemon or lime juice in a quart of warm water."
That's it. The bleach sounds scary to me too, but I'm imagining the chlorine keeps out the gunk, just like in a swimming pool. Check out the photo for a visual comparison: