Green Algae: Green algae usually floats, but sometimes it attaches to walls. Water becomes turbid with a green growth that tends to keep swimmers far away from the water. The pool surface usually becomes slippery and it’s hard to see the bottom, both dangerous conditions for swimmers. Before the green shows up, the side of the pool will develop a slick feeling and the water becomes hazy.

Mustard Algae: Also called yellow algae, it usually appears as a yellow powdery deposit on the pool. Most often found in the shady sections of the pool, it is resistant to even large quantities of chlorine (even 4-5 ppm).

Blue-Green Algae: Also called black algae, you can spot the dime to quarter-sized black or blue-green spots clinging to the pool’s surfaces. Blue-green algae are also chlorine resistant. This algae type forms a layered structure where the first layer can be killed by chlorine, but the bottom layers are protected.