Exactly my point. They are unable to "suck" enough water to fill themselves up, SO, you have to fill them up with water, prior to turning them on. If it's filled 3/4 of the way, it won't work properly, because they "push" water better than they "pull" water. For this reason, they never fill up. They fill to the level they've always been at, because there's not enough force, from the water coming in, to expel the air pocket, and replace it with water. Consider how much water these are supposed to turn over, 350 gph. If it were 3/4 full and able to "suck" don't you think it'd fill the rest of the way? They don't because they're unable to "suck." Once full, they will push 350 ghp, and remain full, because water is naturally replaced, by way of a natural vacuum, as water is pushed out of the canister. The reason they work better, when below the water line, is because they fill up with water, through gravity. This is how they are primed.