Converting a 400-Gallon Hot Tub to a Koi Pond

Hot Tub to Koi Pool: Feasibility and Stocking Limits

A 400-gallon hot tub conversion. Surface area ~162 sq ft (example calc). Stocking formula: 4.5 sq ft per 12 inches of fish body (unaerated). With aeration, capacity increases.

Key Calculations

• Unaerated: ~30 inches total koi body length (e.g., 2–3 koi at 12–15").
• Aerated + filtered: Can support 6–8 small koi, but they will outgrow it.
• Koi need 1000+ gallons long-term. Hot tub is a temporary juvenile setup.

Essential Equipment

• External pump + bead filter or bog filter (hot tub plumbing may not suit).
• UV clarifier for green water.
• Aeration: air pump + diffuser.
• De-icer if outdoors in winter.
• Cover/netting (hot tub walls are steep—escape risk).

Supplier Suggestion

Aquascape Designs for filters, pumps, kits.

Better Use

Goldfish, shubunkins, or orfe (if pond >1000 gal) are more suitable for 400 gal. Koi will stunt and suffer.