Review (5/5): How much waterbed conditioner to use
I have used this product for about 3 years now, and recommend it fully. The only purpose for "waterbed conditioner" is to keep in check the organisms which naturally live and multiply in the water inside your waterbed. They create the bubbles you routinely have to purge from the bed -- if you don't regularly treat the water with a chemical which impedes those creatures' lives, they make many more bubbles and can eventually ruin the inside of your waterbed bag. You only need to use 4 ounces every 6 months, or 8 ounces once a year. I have slept on a waterbed for 40 of my 60 years, so I have enough experience in the matter.
You will know when it's time to purge the air from the bag and add conditioner, because the noise of the bubbles in the bag will begin to bother you and keep you awake. That's the time to purge the bubbles, and add 4 or 8 ounces of conditioner -- the size of the bed does not matter, just add a 4-ounce bottle every 6 months.