What kind of algae is this?

[Watermom]

With a CYA over 100, your shock level is going to be 25ppm. Test at least 2 and 3x or more per day is even better and each time, raise your chlorine level back up to 25ppm with bleach. The key to killing algae is to sustain the high cl reading and not let it yo-yo up and down. Do this until you can go from sundown to sunup without losing more than 1ppm of chlorine. At this point, you can let the cl level drift back down but with that CYA reading, you will have to keep your chlorine between 8-15 ALL the time or your algae problem will come back. Also, brush the pool daily and run the pump 24/7 while you are trying to clear this pool. BTW -- In a 20,000 gallon pool, each 3-quart jug of 6% bleach will raise your cl by just a little over 2ppm.

No more trichlor tabs nor dichlor powder shock for your pool as they will continue to cause your CYA level to rise. It is already WAY high. In fact, you might want to consider doing a partial drain and refill and drain about 1/2 to 2/3 of the water to lower the CYA to a much more manageable level. Do NOT drain the entire pool however.

Sand doesn't need to be changed. Mine is something like 10 years old and doing just fine.

No more algaecide. At this point, just lots of bleach. Also, keep an eye on your pH and make sure it doesn't drop any lower. Anything below 7.0 is acidic and can damage your pool. Watch the pressure on your filter. When it rises 8-10 psi over your clean filter pressure, backwash.

Good luck. Keep us posted how things are going.