Hello, I have a 25000 gallon in ground pool with a 3/4 horsepower pump. Just got a new filter due to a cracked multivalve. Installed a 24 inch Hayward sand filter. It was doing well until we had a bad storm on Monday (at least 3 inches of rain in 25 minutes) and then a power outage for 30 hours. Pool chemistry levels were ok except chlorine. Shocked pool. Another storm arrived on Wednesday. Luckily power stayed on but another 3 inches of rain in short time. All levels ok. Backwashed and drained the excess water. Now we have algae. Brushed and vacuumed yesterday and shocked again last night, still green. My question is, could it be my new filter. My old one was undersized but would manage ok.