New User - Green Water

I had the professionals open my pool this season. Water was crystal clear for about 3 weeks, but can't get the pool people to come back. I've taken matters into my own hands by replacing the Polaris 380 with a Dolphin 2020 and having the water tested at a pool store. After adding the chemicals purchased from the pool store, CL is low, Ph is OK, Alkaline is low and CYA is low - and I can't see the bottom of the 9' deep end. Pool is plaster, 40,000 gallons with DE filter and auto chlorinator using 3" pucks. Also, pool is covered for most of the day, filter runs 12 hours and temperature is maintained at 82 degrees. Help!

We need some water testing result numbers to be able to help. Not just this is low and this is OK, but actual numbers. Also, tell us exactly what all has been added to the pool - ingredients, not just 'shock.' Then somebody here can take a look and try and help.

Yup. What Watermom said.
Get a new pool store test to start, then post:
FC (Free Chlorine)
CC (Combined Chloramines)
TC (Total Chlorine--TC=FC+CC)
pH
T/A (Total Alkalinity)
Ca (Calcium or Calcium Hardness, or Hardness)
CYA (Stabilizer/Cyanuric Acid/Conditioner).

If you are using pucks I'll bet your CYA is through the roof--normal chlorine levels are ineffective when that happens and you need MUCH higher levels of chlorine.

Go start reading on PoolSolutions.com, the sister site and read EVERYTHING there you can access. It's a really good start.

Just wanted to say hello to everyone.

I'm in a similar boat with the OP and last year gave up on the pool service company in my area.... they'd never show up, and the one time they did they sent people who didn't have a clue what they were doing. So I started doing my own work.. and found PoolSolutions which led me to stop buying "pool" chemicals from specialists...

Anyway, this year, opening was fine, water was clear, but all of a sudden the algae has taken over, and it's just May (water is still very cold)!

So I did some research here and then discovered my cut-rate chlorine is only 3%. So I haven't been using enough. I'll get the 5% stuff, try to bring the level up and then test. It's a 20K-gal IG concrete pool...

Finally, wanted to say thanks to the regular contributors to this forum. I just discovered the forum today (despite referring to Ben's site for a year now) and look forward to learning a lot from you guys :)

Welcome! Hope you find the forum very informative.