Christmas Theme
Mini Christmas trees (aquarium-safe), snowflakes, bubble wall with red/green color changes.
Keep decor in for 2-3 weeks max. Over-decoration stresses fish.
Halloween Theme
Pumpkins, skulls, witch hats — available from aquarium brands for safety.
Orange and black color themes work well.
Valentine's Theme
Heart-shaped glass or ceramic pieces. Pink and red theme.
Simple enhancement to existing setup.
Safety Rules
Always remove decor if fish show stress signs. Reset to normal tank.
Never use non-aquarium-grade novelty items. Even "looks safe" can leach toxins.