My bad; Carl's right => I should have looked at your dimensions.

Actually, those readings may not be bogus. The bogus ones look like this:
FC = 5.12 ppm
CH = 266 ppm
CYA = 109 ppm
TA = 115 ppm
pH = 7.42
and come from digital test strip readers that tend to get different results every time you stick the strip in the reader.

Anyhow, assuming the CYA levels are right, the "overdose" of chlorine was just right. Probably better add 5 gallons each evening, till you get a test kit. DPD tablets will NOT work for you, at the level of chlorine you need.

Be sure to brush, too. Algae gets in cracks and crannies. Brushing helps get it out, and expose what remains to the chlorine.

Ben