Clearing Green Water in a Small Patio Pond: Algae Control for Tub Ponds

Overstocking Is the Root Cause

A 50-gallon tub pond with two koi, two black moors, and a pleco is severely overstocked. Koi need hundreds of gallons. The fish load fuels algae blooms.

Light and Color

Light blue liner reflects light, promoting algae. Dark interiors help, but in Florida, a dark exterior overheats the water. Keep the outside light; paint the inside black if possible.

Immediate Steps

Rehome the koi. Add a small biofilter (sponge filter with air pump). Introduce floating plants (water lettuce, frogbit) for shade. Use barley straw or a peat bag. Do 20% water changes weekly with dechlorinated water.

Long-Term Fix

Upgrade to a 200+ gallon pond or keep only a few small goldfish. A balanced system clears itself.