Invisi-ghoul

Invisi-ghoul is the sixth mini-game, a conveyor-belt level unlocked by completing Adventure Mode and three other mini-games. It takes place in the Pool area at night with all of the zombies invisible. Items from the zombies, such as ice trails from Zombonis and Buckets and Roadcones falling off from zombies, will be visible for a short time.

Strategies
The garden rake is very helpful for this level, as it provides you with time to set up your defenses, so you can have weapons in every lane to protect from possible zombies.

Listen and look for small details for clues as to what zombies are on the screen and where they are. Don't turn off your sound, as the sound is important for identifying zombies. It is possible to tell what or where a zombie is when the projectiles hit. For example, Conehead Zombies make a dull thudding sound when hit by peas and Buckethead Zombies make a metallic clang. Peas cause a splatting explosion that lets you identify sounds. If a pea hits a zombie near the beginning of the lane, and the next pea hits much farther into the row, the zombie must be a fast zombie, either a Dolphin Rider Zombie, Flag Zombie, or Jack-in-the-Box Zombie.

Save your Squash for either as a last resort or to kill Zombonis. The Zombonis, if ignored, can destroy an entire row without much help, paving the way for a Zombie Bobsled Team or other zombies right behind.

Kernel-pults should be spread across all rows evenly. The Butter will not only freeze the targeted zombie in place but will also signal you where a zombie is for a few seconds. Only double up Kernel-pults if you have more than one in every lane.

Some zombies also alert the player that they are coming by making distinctive sounds or actions. Jack-in-the-Box Zombies make music, Dolphin Rider Zombies make a distinctive cry before entering, and Zombonis have an engine noise and leave an ice trail. In addition, if a zombie enters the pool, there will be a small splash, which alerts the player there is a zombie entering the pool.

Even though the zombies are invisible, you can see their headwear (if they have any) and arms come off when they have lost enough health, and their heads visibly fall off when they have been defeated. With the use of the Ice-shroom, ice forms around the zombies' feet, making you not only aware of their location, but also giving you time to set up defenses in certain lanes. But keep in mind that the Ice-shroom only slows down Dolphin Rider Zombies with its Dolphin and does not work on Zombonis. Also, ice will not form around the zombies' feet in the pool. In the Xbox Live Arcade, PlayStation 3, and PlayStation Vita versions, the game is made much easier because all zombies still shake the bushes when they enter.

Trivia

 * The picture for Invisi-ghoul on PC and Nintendo DS is a picture of a splash in the pool, a great way to know Ducky Tube Zombie or Dolphin Rider Zombie is coming.
 * This mini-game takes place in the pool at night, but there is no fog. The only other mini-game that takes place in the backyard at night with no fog is Dark Stormy Night.
 * If you don't count cancelled mini-games though, this is the only minigame to be a fog level with no fog.
 * When a zombie loses its headwear, arm or head, it will become visible.
 * Invisi-ghoul is a portmanteau on "invisible" and "ghoul".
 * Even though this level takes place in the pool at night, there is no fog, as the zombies are invisible, which would render fog useless.
 * This is one of three levels in the game with no visible zombies at the start of the level. The other parts are Level 5-10 and Dr. Zomboss's Revenge.
 * The first zombie coming is usually a Conehead Zombie, contrary to most levels where the first zombie coming is a normal zombie.
 * However, sometimes the player will get two normal zombies to start with, instead of one Conehead Zombie.
 * Sometimes, in the Nintendo DS version of Plants vs. Zombies, there is a glitch that when a Zomboni appears, an ice trail suddenly appears, and the player plants a Squash right in the middle of the Zomboni, the Squash might not squash the Zomboni, instead it stands right there, on the ice trail, or hitting zombies behind it.
 * This mini-game possibly explains that the fog on fog levels is from a fog machine, for, if it were to be from natural events, the fog would still appear.
 * When an Ice-shroom immobilizes all zombies on the screen, there are ice crystals found at the zombies' legs, letting players know where they are.
 * The Kernel-pult's butter also has the same effect.
 * The first two plants are always Peashooter and Ice-shroom, in that order.
 * It is possible that the Jack-in-the-Box Zombie will not appear in this level.
 * High Gravity has the same logo.