Possible Independent Variables

air movement (wind speeds, wind direction)

chemical concentration (low to high concentration)

chemicals (none vs. household chemicals)

dissolved gases (oxygen, carbon dioxide)

environment (simple, complex, large, small, maze, pen)

foods (types, enhancers, freshness)

light color (red, orange, yellow, green,  blue, violet)

light intensity (absence or presence; high to low intensity)

light quality (incandescent, fluorescent, natural)

nutrients (natural vs. synthetic; commercial brand A vs. brand B)

salinity (fresh vs. saline, low to high, sodium chloride vs. other salts)

sound (volume, pitch, rhythm, music)

temperature treatments (cold to hot, pattern of exposure)

water pH (acidic, basic, neutral, pH 0-14)

water source (tap, distilled, bottled, stream, lake, ocean)