These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: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.
Iit gets worse when in tank 4, it attacks alongside Balrog.
The Type-D alien, AKA Destructor, can be this for first-time players. 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.
Goddamned Bats: All of the aliens unless you have Blip, since their attacks locations are unpredictable