Sometimes you added to much Most times it has to do with existing water conditions. Minerals,hardness, existing organic mattter. It is not always the brand of algecide. Add a defoamer. The cheaper algaecides are quaternary ammonias, also known as "linear quats". They are commonly found in large, diluted gallon jugs, and also smaller concentrated quart sizes. Most stores like Walmart and Kmart carry them because they are the cheapest. Linear quats will foam - it's normal. In fact, if you don't notice any foaming at all, you probably don't have enough to kill algae. Linear quats don't actually kill algae (only chlorine does that). They work by stripping the cell walls of the algae, which allows the chlorine to kill. Look at the long chemical name on the bottle of algaecide, it will begin with the word "Alkyl". That's a linear quat. The more expensive (and better) algaecide is known as polyquat. It is commonly sold as "Algaecide 60", or some similar name. Often times, the big stores don't carry this type because it's more expensive. Best place is to order it from Leslie's Pool or In The Swim online. Polyquats do not foam and have no side effects at all. They also are highly viscous and act as somewhat of a clarifier as well. Polyquats work similar to Linear Quats in that they disrupt the cell function/walls of algae and allow chlorine to kill it. The Polyquat 60 is the algaecide that I recommend. Even though you didn't ask...the other highly effective algaecide is Copper. Algae cannot grow with even a relatively small amount of residual copper in the pool. Be careful if you decide to use it, however. Copper can (and will) stain concrete pools. That's not really a concern on vinyl liner pools. And have you ever heard the old saying "chlorine will turn blonde hair green"? Well, chlorine does not affect hair color. Copper does. If you have enough copper in the pool to be an effective algaecide, you probably have enough to turn blonde hair green. So yes...it is very effective....but it also has some side effects. Overall....Polyquat is a bit more expensive, but it has no side effects. Go with that one.