No need for algaecides--you just need to keep the chlorine at appropriate levels for your CYA. With a CYA of 55, that means keeping it in the 5-10 ppm range at all times. Letting it get below 5 ppm for any length of time will give the algae a chance to come back. And as you can see, it's much easier to keep the levels up than it is to clean up the mess if you don't