I have the black check valve from Stenner. I have not had any water back flow into my chlorine tank. I really like the idea of the tank buried in the ground for a cooler temperature, but I did not put mine in that way. The 10% (Pool Essentials $3.64/gal) concentrated bleach degrades quickly. Mine was about 2 months old (stored in the garage) and it degraded from 10% to 6.5%. My next bleach purchase was the 6% cleaner bleach from Walmart for $1.77/gal. Looks like the 6% is cheaper and more stable. The picture shows bleach stabilty. There was no copyright on the picture so I think it's safe to post. I set the Stenner pump to add 2ppm and then measure the actual change in FC. Since the relationships are all linear, you can just prorate your Stenner pump run time up or down to get the desired change in FC. Example: running for 120 minutes gives 4ppm increase in FC, so run for 60 minutes to get 2ppm.