Challenge Mode is the hardest mode in the game. The player must fully understand the game in order to beat it.
Challenge Mode is for experts of the game. The goal for this mode is the same as Adventure Mode: collect three egg pieces to finish the level. However, there are some twists, which pretty much makes Challenge Mode challenging.
One, every 4.5 seconds, each item that is already opened will increase its price by 1% of its original price. Example, a guppy costs $100. 1% of it is $1. So, it's price will keep increasing by $1. Same goes with other items. A $200 food quality upgrade will keep increasing its price by $2. A $300 food quantity upgrade will keep increasing its price by $3.
Two, once an egg piece is bought, all items will reset back to its original price, except for the egg piece itself. Example, if you bought an egg piece and its price is $7000, it will stay like that. After the cooldown of about 15 seconds, that $7000 will again constantly increase.
Three, the longer you take to finish the level, the peskier those aliens are gonna be. The longer you play, the harder they get. They'll increase in difficulty and in number. Too many aliens can make finishing the level impossible. So speed is an important factor of Challenge Mode. Take too long, you might not make it.
Another thing, after five minutes, purchase options that are still closed will be automatically opened by the game.
Each tank rewards differently.
Tank 1 - 2000 Shells
Tank 2 - 5000 Shells
Tank 3 - 10000 Shells
Tank 4 - 20000 Shells
Also after completing a tank, the player will be rewarded with a story about a pet or an alien.
Tank 1 - Story nos. 1-5, 21, 25, 26
Tank 2 - Story nos. 6-10, 22, 27, 28
Tank 3 - Story nos. 11-15, 23, 29, 30
Tank 4 - Story nos. 16-20, 24, 31, 32
After getting all the stories noted, complete any tank to get Story No. 33. After doing so, the player will be given the coveted Golden Trophy.