Hi Angi
Okay I hate to break it too you, cause I can tell you are frustrated.
You are being pool stored.
Black Algae rarely and as far as I can tell in over 2 years on this forum is
undocumented in an Above Ground Pool, meaning, it doesn't happen. So if your pool store sold you that chem, they either don't know what they are doing or they are just taking your money.... (same thing happened to me before I found TFP.) Black Algae is small, eraser size dots that usually grow on rough surfaces like plaster pools, and they spread in a circular motion. Black Algae isn't free floating and it doesn't look like a typical green swamp that some pools become in the spring. You probably just had good old fashioned green algae - and guess what kills that? Chlorine, and plenty of it.
If you added the CYA to the sand filter, and then backwashed, it's likely you washed it out the backwash hose before it dissolved....
What day did you add it with the nylon?
How are you testing? If it's with strips, they are not very accurate and you may have some CYA in there, just not enough to allow the chlorine to do its job. Also are you sure it's not Total Chlorine is 10? Are you sure it says the Free Chlorine is 10?
With BBB and shocking your pool, you add enough bleach to reach your shock level (with a cya of 0 that would be FC of 10) and HOLD IT THERE with frequent bleach additions, until the FC holds overnight, the CC is .5 or less and the water is clear.
You need to run the filter 24/7 until the pool clears. Read how to shock your pool,
here.
With sand filters, it can take a long time to clear up dead algae, if it's dead. but I suspect since your CYA is zero you still have a ways to go.
I highly advise you to get your own test kit so you can take total control of your pool, and no longer be subjected to bad advice from the pool store or convinced you need unneccessary chems.
Read about the test kits
here.
Hope this helps - if you have any other questions or need more help, ask away.
