Flushing is a problem with ponds like yours and adding lime. My 2 cents - with a 5.4pH I would add a bunch of lime when flushing is at its smallest if I could afford too. That pH is hard on fish and water quality - see chart 2 noting wide swings in pH cause stress on fish.

[Linked Image from forums.pondboss.com]

[Linked Image from forums.pondboss.com]