Aliens: Sylvester: Just keep him away from your fish population and you'll be fine.
Balrog: Second basic alien so basically same advice except he's a bit better at eating your tank than Sylvester so be a bit more careful and always keep him in the corner opposite both side and corner of your fish (Fish: Top Right corner Balrog: Bottom left corner)
Gus: He doesn't die to lasers but he dies to overeating. "G" stands for Gus and glutton and that is what he is. Keep him away from your fish at all costs unless dying has become good somehow. Never let him near your fish because usually he gets it in the end.
Destructor: A robot with missles...Must be a scrapped boss for some game I never got. But at the same time a cool boss in a game full of fish. He is confined to the bottom where Rufus can get him easily and just fires off missles to kill your fish. Shooting the missles blows them up and saves a fish. Not as hard as Gus and sometimes not even Sylvester is as easy. Just wait until he fires his missles to blow them up not try to blow them up just before they leave their holders. I'm not even sure it's possible so just wait until they lock on and then focus on them before blasting Destructor himself.
Psychosquid: Probably the hardest alien to kill. He's fast, he's deadly, he's even got an invincible form and guess what? He'll abuse these factors every time you face him. Capitalizing on speed to wreck your tank before you can blink (do it really slowly) and making you start all over again. Shoot him away from your fish unless you like the game over message. After a while he'll turn blue, drop his tentacles full of weapons, and will actually be healed if you shoot him now. A message will appear stating: "Stop Shooting Alien Recovers Health" so just wait until he changes back before hammering him again. He'll occasionally manage to get Invincible form twice before dying, a heavy pain in the butt for most players.
Ulysses: Pretty much a more deadly Destructor. His 2 laser balls however will change into a hard hitting owning laser that goes through multiple fish until it hits the shooter or a wall. So show a lot more caution and you will be fine.
Bilaterus: Hey a two headed snake of death. You have to kill both heads and get the one diamond without dying. Everything on him kills a fish: spines and heads will kill your fish fuuuun.... KEEP HIM AWAY FROM YOUR FISH AT ALL COSTS!!!!! He takes a lot of skill and luck to beat but eventually he will fall and you will probably have mastery of all aliens in the game or so you think.
Cyrax: One hard alien. He summons all the previous aliens at any given moment but one at a time except for the Tank 1 aliens. He also summons Mini-Sylvesters more commonly that can die in one hit but can eat a lot of pets before you see him. Shoot him until any other alien appears in which case kill that one first otherwise you risk total destruction. Just keep hammering him in between alien attacks and he'll eventually fall before you. Your prize? 5,000-25,000 shells depending on how many times you've done this. And on the first round the best pet ever: Presto!!!
Sandbox: Press Z, X, C, V, B, N , and M to summon an alien and kill it.