These are great! I have three dogs and recently moved into place which the owners were very concerned about their lawn. I kind of took a leap of faith and told them not to worry about it, I would use the rocks to prevent the yellow spots. Thankfully they didn’t let me down! They MUST be used right. I personally bought two 2qt pitchers and put a pack of rocks in each pitcher, if you read the package the rocks are good for 2qts of water, which is perfect. I rotate the pitchers, one pitcher a day, making sure that they’ve had at least over night to soak. The rocks aren’t magic, they need about 12 hrs to filter the water. Just make sure to read the directions thoroughly and they should work! Three dogs, no yellow spots. Awesome.