* AnnoyingVideoGameHelper: Blip in challenge mode. He opens everything in the shop early, making everything go up in price early on. This can break the game against your favor.
** Breeders. Once you get enough of them, they'll actually produce too many guppies.
* BreatherBoss: Destructor. While he can be a DemonicSpider if you don't know how to destroy his missiles, once you learn that, he becomes a joke. He's also a big part in [[spoiler:getting a NoCasualtiesRun at the final battle.]]
* CrowningMusicOfAwesome: Some of the soundtrack in the game is rather amazing.
* DemonicSpiders:''Psychosquid.'' (aka the Type-P alien) It has very high health, moves faster than all of the aliens, eats multiple fish at once, and after some time, he goes into a defensive mode where he regains health if you shoot him.
** It gets worse when in tank 4, it attacks alongside Balrog.
** The Type-D alien, AKA Destructor, can be this for first-time players. [[GuideDangIt There's no indication you can shoot the missiles it launches]], and even all the offensive pets plus a max weapon can't kill it before it launches a round of missiles. So you'll be spending a lot of money to replace your fish before you learn how to destroy the missiles.
** Bilaterus. For one thing, [[AmbiguousGender he/she/it]] has to be killed ''twice'' to stop invading the tank. His spine also can eat fish, giving him a large hitbox for fish to be eaten by.
* DoNotDoThisCoolThing: Despite the game suggesting players to never shoot blue Psychosquid at any cost, it's actually a very good idea to shoot him when he's dangerously close to a fish you like.
* EnsembleDarkhorse: Blip. He can predict when your fish are hungry, where an alien will appear, and how much health an alien has left. That's all really cool, but his selling point is opening everything in the shop early. This allows you to prepare better for the first alien by buying extra lasers. However, he's a very bad pet in Challenge Mode, as him [[BlessedWithSuck opening the store early causes everything to go up in price early on.]]
* FridgeBrilliance: Carnivores eat guppies, which cost 100 dollars. They drop diamonds, which give you 200, enough to buy two guppies. Same with ultravores, but multiply both values by 10. Coincidence? Unlikely.
* GameBreaker: The Amp/Angie/Presto combo can give you an enormous sum of money with ease, as you can turn Presto into Shrapnel to create more money easily, then buy a ton of guppies to zap with Amp, then transform Presto into Angie to revive them. Adding Walter into the mix makes it even better, as you can charge Amp up earlier and zap the fish away more often.
** Basically every alien, even the harder ones like Psychosquid and Ulysses become this if you have Gumbo.
** Gus, the type-G alien. You can't hurt him with lasers, so you basically have to feed him to death. The problem is that when he first appears, you probably don't have a food quantity upgrade yet, so you can't just mash the left click button on him like other aliens.
** [[https://youtu.be/QTrFtrRZVCs The "Alien Apporaching" theme]] is incredibly chilling, especially for a game as innocent looking as this. It sounds like something directly from a slasher film.
** Psychosquid really lives up to his name. He's a big, red squid with dangerous weapons in his tentacles, and eats fish much faster than anything else in the game.
** The ending to 4-5, the level before the FinalBoss. The egg hatching animation goes as normally... [[spoiler: and then a freaky alien appears out of nowhere to stop the animation (With a terrifying EvilLaugh to boot)! And you have to fight him to get the actual egg!]]
** Said FinalBoss, for that matter. [[spoiler:Your pets are all dropped into an eerie tank. Then Cyrax, the final boss, appears. None of your pets can do anything to fight him, and the aliens can eat them. Going through the entire game knowing your pets are safe from aliens just to find out that they're at risk in the end is horrifying.]]
** While Rhubarb is this In-Universe, most fans see Nostradamus as this. He produces snot which barely gives you any money, and guppies can eat them and turn into another Nostradamus. He also stops aliens via a deafeningly loud sneeze, which feels more like delaying the inevitable than an actual help.
** People are also not very fond of Gash, because he eats guppies, which includes adult guppies and [[spoiler:''royal guppies.'']]
* SelfImposedChallenge: [[spoiler:In Sandbox Mode, summoning wadsworth, then summoning Ulysses (The Cyclops alien), then summoning a whole bunch of non-guppy fish. The goal is to try to get Ulysses's orbs to kill all of the fish. You can also add to this by putting in Angie.]]
* SidetrackedByTheGoldenSaucer: Justified, since the shells you get from the Virtual Tank can help you buy extra pets for the main game.
** [[spoiler:Sandbox mode might also count.]]
* TearJerker: The end credits song,[[http://www.youtube.com/watch?v=w9scA_0ZKbY Lullaby]]. It's just so beautiful...
* ViewerSpeciesConfusion: It's actually surprisingly common for people to mistake Psychosquid for an octopus, despite the fact that he's clearly a squid because of him having 6 tentacles, a coned head, and his name being Psycho''squid''.
* TheWoobie: Clyde. The sole reason he works for your tank is because he needs the money for his mother, who's in the hospital.