Believe me,I know which ones to mix and match thee days after many fights and some killings back in the day.
Have had 2 Oscars over 12 inches long.....had an Arrowanna that was 27 inches long.....had to sell him to a Chinese restaurant that had a 300 gallon tank....all I had at the time was a 100 gallon tank which got too small for him.....he was looking at my expensive fish like they were on the menu....I used to feed him live mice.....Google Arrowana if you`re not familiar with the fish.
I bought him as a baby with an egg sack which they feed from when young still on him for 6.99 at 4 inches long.... and the Chinese dude gave me 100.00 when he was 27 inches long.................tha nks buddy. 
Another aggressive fucker is the Red Devil and a Texas Cichlid can be mean too and Green Terrors.
My monster fish days are all gone................my only Cichlids these days are Angel fish................my Jack Dempsey killed my Chocolate Cichlid and that`s when I quit the monster fish obsesion...........why get them if they are just gonna` beat the fuck out of each other...............and kill each other?
Great hobby though................it helps to keep me sane..... and I`m legit crazy....got the papers to prove it..... so I need all the help I can get. LOL 
The arrowana looks cool, but don't think i'll get one anytime soon.
I've had Jack Dempseys, beautiful fish. Had plenty of Firemouth Cichlids.
I still have a 100 gal with Cichlids, but they are very selective breeds. I buy them from a guy in Arizona, they are expensive but never die. Some of them are over 8 years old.
I like the colors they get when the water is right and the food works. Certain times of the year they glow.
There is a 10" ghost knife fish in there too, but he has a lot of places to hide.
The only fish that can legitimately swim backwards in a straight line.
Looks like an eel.
I have this awesome castle i bought like 15 years ago, the knife fish fits inside perfectly.
The cichlids seem to leave him alone, plus he's really fast.
I've had tanks since i was an infant. Parents were big into animals. I grew up on a farm.
Just watch the fish for a while, it does have a calming effect for a short period of time.
