I'm trying to figure out what kind of fish I want to get for my tank. I'm thinking about getting piranhas, I want dark mean looking type of fish for my tank, any suggestions??

I don't know anything about Piranhas or any other type of predator fish, but I have heard about Exodon paradoxus, a smaller relative of the Piranha. If you wanted to find more about dark mean fish you could look at the Predator forums here. Alternatively, with 90 gallons you could do a lot of other stuff. I would recommend (if you have some experience) keeping Malawi cichlids, which are probably the most colourful freshwater species and are still pretty mean. That being said if you are interested in Piranhas for their diet Malawi cichlids would probably disappoint you. As far as I know few of the more colourful/popular species get big enough to eat mice and goldfish and in fact some (Mbuna) are even vegetarian I believe.

On a side note you may also want to check which species are legal in your state, especially if you decide to go with a predator and mail order.

thanks for the info , i to be honest, I just want to set up a really awesome tan, I want it so that people are put in awe when they see it lol, so I'm just looking for suggestions of what type of fish I should put in there or how I should set the tank up. I'm looking for big fish that other people and myself will love.

Thanks!

tank* not tan hahaha

If your looking for a tank with a "WOW!" factor I'd definitely have to suggest a tank with Malawi cichlids, an all male perhaps. They are truly beautiful fish. I find that as a display tank I have yet to see a Piranha setup that works for that. All the ones I've seen are bare tanks with maybe a fake plant or two and then the, in my opinion, homely fish just swimming around. Which is still super cool, but the feeding frenzy thing doesn't happen all time, I don't think. If you did want a really unique fish to keep you could look at Bichirs. They are a type of ancient eel-esque fish (kind of, they are totally unique) and some species (Ornate) are very beautiful fish. Here's a link that has some really good stuff: Bichirs (To the Moderators: I hope I'm not infringing on the "No Advertising" rules, from what I could tell the site is not a forum, or selling anything.)
I believe the Ornates are larger than the Senegalese (I believe those are two common species), but I think you would have enough space for one, possibly two. Again, I'm no expert on large predator type fish.

Get a Red-Tailed Barracuda...Great fish, fun to watch eating...

Get a couple oscars. With black sand and fake volcanoes. That'll look mean!

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16g-2 guppies, 1 bolivian ram, 3 neon tetras,2 khulis
55g-1 red tail shark, 1 rubberlip plecos, 2 banjo cats,
2 silver dollars, 6 white clouds,

I agree with SeanM... bichers are cool prehistoric fish

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10 g planted community, 10 gallon danio/RCS tank, 10 g ghost shrimp grow out tank, 10 g blue pearl shrimp tank, 75 g in the works.

That would be a cool look...lol