I want a cichlid that I can buy at petsmart or a petstore that is around 6 inches or under. I want a really cool looking one or really aggresive one. Any ideas?

Jewel Cichlids are good. Moderately aggressive. Some of the Africans are good too.

definetly blue electrics, but they are more than 6 inches. so i would go with the jeweled chilchids.

African chiclid grow to about 6 inches. Aggressive.

african cichild or jewel cichild.

head

Well, the most aggressive (territorial) Cichlid I have had experience with is the Wolf cichlid, also known as Dovii. They won't make the size cut (they can get really large), but they are truly ferocious, even attacking their keepers hands and ramming the glass. pets question and answers,www.5d2d.com
Also too large for your question, but an exceedingly tenacious hunter are the 4 or 5 species of cichlids known as Peacock Bass. They are only territorial during spawning, but when they feed they are known as pursuit predators,once they lock on to a prey fish, they won't quit until it's caught.
I can't think of a truly aggressive cichlid in the size range that you ask for, but there are many territorial types that behave as if they were aggressive because they are kept in tanks that don't allow for their normal territory size.
Almost all of the cichlidae are interesting in one or more facets of their life styles, many have interesting breeding habits like African riverine types Jewelfish or Pelvichromis pulcher ,aka purple kribensis. There are lots of others, but the 6" size limit cuts way down on the numbers.

60 years of fishkeeping.

1. Wolf Cichlid
2. Red Tiger Cichlid
3. Yellow Jacket Cichlid

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4. Jaguar Guapote
5. Three-spotted Cichlid

me

If you are looking for a cool looking aggressive cichlid...I would try Malawi Mbuna cichlids. They will stay under your 6" mark and are pretty aggressive and have awesome coloration. Although not nearly aggressive as the other fish mentioned like the peacock bass and wolf fish, which will both grow WAY bigger than what you are looking for and would require a minimum of 125 gal. Pseudotropheus Polit is a very aggressive small mbuna that has awesome coloration, when mature a male will turn from solid brown to an all white with almost blue sheen body with a black mask. Mbuna are the most aggressive of the African cichlids and the pseudotropheus being in general the most aggressive of the mbunas. Convict cichlids have decent coloration and are aggressive, but if you are looking for the coolest fish with an aggressive attitude my route would be Pseudotropheus Polit, google search and check them out. Ihave several and the male, who is about 3 1/2" is the boss of my 80gal tank, and all the other fish know it including several fish in the 6"-8" range. How big is your tank and are you only looking to put one fish in it?