With a CYA of 50, you need to shock your pool up to about 15ppm and try and hold it there. You can use cal-hypo, although with a vinyl pool, you do not need calcium and large doses of it sometimes will cloud up your pool pretty good. I do not know how to gauge the amount of cal-hypo to use. Maybe the label will give you some indication. We typically just recommend that people just use plain, unscented household bleach for their source of chlorine. It is 6% sodium hypochlorite and will not cloud the water. If you ant to go that route, add 6 gallons slowly to the skimmer while the pump is running or slowly in front of a return jet being careful not to splash it on you or the liner.

At least twice per day, and more often than that is even better, test the chlorine and each time, add enough bleach to bring the cl back up to 15ppm. In a pool this size, each gallon (4 quarts) will add 2ppm, so use that to help you figure out how much to add.

Run the pump 24/7 and backwash anytime the filter pressure rises 5-10psi over your clean filter pressure.

Don't add the yellowrid. It will further complicate things and you do not need it. If it is unopened, see if the pool store will let you return it.

All you need to clear algae is sustained high cl levels.

You'll need a good test kit if you don't already have one. The one we recommend is the Taylor K-2006 or 2006C (same kit, larger bottle of some reagents). If you buy it through the Amazon link in my signature, the Pool Forum makes a little money on the sale which helps us keep this form online. Only buy if the seller is Amato Industries, however. Some other sellers are substituting the K-2005 which you do NOT want.

In the meantime, you can use a dilution method to test higher cl levels than your kit will be able to measure since you're going to be shocking up to 15ppm. More info about that can be found in the following link. It is a temporary aid until you can get a better kit.

How To Test Without a Good Kit

Hope this helps. Keep us posted how things are going and glad to hear you didn't fill from a well!!