One gallon of 12.5% liquid chlorine will add 4 ppm of chlorine to 30000 gals of water. And liquid chlorine is what you want to use when fighting active algae. I personally think it's the best choice for the daily sanitizer also.
With actively growing algae you need to shock the water and maintain the shock level until the algae is dead. This is a process known as slamming the pool.
The Chlorine shock level (as wel as the everyday sanitizing level) is dictated by the CYA level in the water. The shock level for a CYA level of 55 is 22 ppm, not 10 ppm. You will need 5 gallons of 12.5% chlorine to raise the chlorine level to 22 ppm.
When you are getting the chlorine to do the initial shock, pick up several additional gallons because you're going to be adding chlorine several times during the day.
Backwash the filter before starting the process (unless you just backwashed it) and with the pump running, slowly pour the chlorine in the pool in front of the return in the deep end.
Let the pump run continuously until the process has been completed and the water is clear.
After adding the initial 5 gallons of chlorine, wait 3-4 hours and get a fresh chlorine reading. Subtract the free chlorine number you got on that reading from 22 and add additional chlorine to bring the level back to 22 ppm. NOTE: 32 ounces of 12.5% chlorine will add 1 ppm of chlorine to your water.
Chlorine readings should be taken several times a day but not more often than hourly and not less than twice daily. The last reading should be taken as close to sundown as possible and the first reading should be taken as close to sunup as possible. After each reading bring the chlorine level back to 22 ppm. The amount of chlorine needed after each reading should be less and less.
You should vacuum and brush the pool daily during this process. The water should look a little clearer each day. You may not be able to see a difference for the first couple of days. That's because it takes a sand filter longer to filter out dead organics than a cartridge or DE filter. There are a couple of ways to speed clearing the dead algae from the water. You can add a little DE to the filter or you can get a skimmer sock at the local pool store. The DE added to the filter will catch smaller particles than the sand can catch. The skimmer sock will also catch smaller particles. If you decide to do that, do one or the other, not both. If you decide on DE, check the filter pressure frequently and backwash when needed. If you go with the skimmer sock, check the sock frequently. The dead algae will coat the sock and that will reduce water flow to the pump. If the flow gets too low it could damage the pump motor.
Stop the slam process when the following have been met:
1. Pass the Overnight Chlorine Loss Test
2. Combined Chlorine is .5 ppm or less (Calculate this be subtracting free chlorine from total chlorine)
3. The water is clear
When you reach the point where you are only adding 2-3 ppm of chlorine for the whole day, you are ready for the overnight chlorine loss test.
To do that, take the evening chlorine reading as close to sundown as possible. If the chlorine level is 19 ppm or higher, don't add any chlorine, just note the reading. Then take the morning chlorine reading as close to sunup as possible. If the chlorine level is 1 ppm or less than the previous evening's reading, the chlorine loss test has been passed. If the Combined Chlorine is .5 ppm or less and the water is clear you are done with the SLAM and you can gradually let the chlorine level drift back down to the normal chlorine level. (Which should be a bare minimum of 4 ppm for water with a CYA level of 55. And ideally you should maintain a chlorine level of 6-7 ppm in water with a CYA level of 55). If the chlorine level difference is more than 1 ppm , bring the level back up to 22 ppm and continue the process.
It would take less time (and be easier and more accurate, (imho), if you do the chlorine readings yourself. If you decide to do this you will need a FAS/DPD chlorine test kit.
You may not know this but the Alkalinity Up product that you a premium price for at the pool store is nothing but ordinary, everyday, household baking soda. I use that when I need to adjust total alkalinity.
Phosphates- Phosphate numbers are only important if the pool has a problem...like algae. Phosphates are a food source for algae, nothing more. The difference between a low number and a high number is the amount of food available. But If you keep the water algae free it doesn't matter what the phosphates reading is.
It's analogous to wood being fuel for a fire. Doesn't matter how little or how much wood you have if it doesn't come in contact with a flame.
With algae, even if you remove all phosphates from the water you still have to kill the algae that's present and remove it. But if you kill the algae first you don't need to spend the additional money to remove phosphates.
Pool stores are needed and can be extremely helpful. But the bottom line is they are there to sell you something. That's why they offer free water testing.
Good luck and if you have any questions let us know.