Milky Indoor Pond Water and White Slime: Oxygen Deficiency

A 700-gallon indoor pond with 2 koi and 5 catfish has milky water and white slime on liner. George diagnoses low oxygen:

• Check filter cleaning, water return splash for aeration.
• Add air stone.
• Assess stocking: koi may be too large, too many fish.
• Do gradual water changes over several days.

A nearby 400-gallon tank with feeders has no issues, confirming filtration/aeration difference.