Acid washes are great for pools if the surface is made from plaster. Over the years, a pool may experience an algae bloom here and there. When this happens, the microorganisms in the algae can stain the plaster which will create a yellow or greenish look in your pool. This can become frustrating because no matter how much chemicals you add to the pool or how much brushing is performed, the algae stains never go away! The combination of acid and water will result in a fresh new looking surface.