Hi jimmerz,
I just finished putting up my new (used/rebuilt by me) 21' with a deep center, so were close, with my calculations and using the pool calculater with my additions I'm somewhere about 12,750 gals +/- 400 gals.
Read up, and post a full set of test # using your best tool you have (the TF100 test kit)
First thing like jasonlion said, get the pH to 7.2-7.4 range.
Use this in Pool School and the Pool Calculator:
pool-school/pool_water_chemistry
Get some CYA and use the pool calculator here:
http://www.thepoolcalculator.com/
Figure how much CYA you need to get to 40ppm (my opinion to do, having a lot of sun and heat here in FL.)
I'm at 50ppm.
Put it in a sock and get pump running and hang over return.
3-5 days it will show up in your test (if it's hot out)
Go to pool CYA chart and look for chlorine level you need for 40 cya, use min # for now.
pool-school/chlorine_cya_chart_shock
At 40ppm CYA, get 3ppm of chlorine in the pool and have some swimming fun for now.
After you posts your test result we can help advise what's next!
Enjoy...
I going in my pool now this talk is getting to me!
Chuck