Hi, spacechimp,

Welcome to the forum!! What type of pool do you have (liner, gunite, fiberglass?) and what type of filtering system (sand, DE?) Also, what is your normal means of chlorination? (Need chemical names, not brand names).

In a pool your size, with a CYA of 55, you're going to need to get your chlorine level up to 20 to attain "shock level". When you added 6 gallons of bleach, it only increased your chlorine to about 10, and with the algae, it didn't stay there long. You're going to need to add 12 gallons of 6% bleach to attain the 20 ppm mark, and then HOLD IT THERE by testing and adding as much bleach as you need to get back up over 20 ppm. You need to do this at least 2-3 times a day, but the more frequently you test and add chlorine, the faster your pool will clear. In a pool your size, each 2 1/2 quarts of bleach will raise your chlorine by 1 ppm, so you can use that as a guide when figuring out how much more you need to add each time to get up to 20 ppm. You need to hold this level until 1)the water clears up, 2) your CC (combined chlorine) is 0, and 3)you're not losing any chlorine when testing the pool at sundown and again at sunup. When those three conditions are met, you're done shocking--you can let the chlorine drift back down but never less than 5 ppm--with your CYA at 55, you're going to have to maintain your chlorine at 5-10 ppm to keep the algae away.

Keep your filter running 24/7 while this is clearing up, cleaning the filter as your pressure indicates, and brush the pool at least once daily.

You can read here on how to force your test kit to read the higher chlorine levels :
http://www.poolforum.com/pf2/showthread.php?t=10073

However, you'll find it much easier to control your pool if you'll invest in a good comprehensive test kit. We like the Taylor K-2006, which you can find here:
http://www.amazon.com/gp/product/B0002IXIIG?ie=UTF8&tag=poolbooks&linkCode=as2&camp %20=1789&creative=9325&creativeASIN=B0002IXIIG"><b >Ta%20ylor%20Complete%20FAS-DPD%20Pool%20Water%20Test%20Kit%20K-2006</b

Janet

Edit: Ben is fast this morning!! But I spent time typing it out, so I'm leaving it......