I saw mattlee's Oscar (Keith) first hand in the flesh before he got rid of him, and he was mahoosive. Easily 14" tip to tail, this to me would be ill-advised to keep in an 18" wide tank. As a general rule i like to keep fish that will only attain an adult size which is half the width of the tank, no more. It gives the fish room to swim, and explore their surroundings as they would in the wild. Going in and among rocks ect. In a 18" wide tank a fish of this size will barely be able to turn around on itself, yet alone navigate obstacles. Thus why alot of peolpe say an Oscar aught not be kept in anything less than a 6x2x2 long term, which makes sense to me. Your average Oscar may attain a total length between 10-13" in general (i believe), so a tank that is 24" wide really is about the minimum i'd concider.
There are plenty of other fish that are far more suitable to what is essentially, a standard 4ft tank. If i was to pick one out from meguro's post it would be the Cupid Cichlids. They are a part of the Gymnogeophagus range, more commonly known as slightly smaller geophagus cichlids. They have bags of character and attain a smaller size. Cupids get to about 6" i believe. I had one, he was the last one left in a shop tank and he was there for weeks. I took him home, and it was easily one of the best fish i ever bought, gave me hours of fun. I found mine to be so inquisitive and very bold, not at all shy.
If your tank is 18" a fish that grows to about 9-10" is probably your best bet if you want something bigger. But with bigger fish you get less in a tank, so a degree of compromise on size is best i find, that way you get more of an interactive community/group. You mention you like the Jack Dempsey, so i feel obliged to say my part on this wonderfull fish. Males can attain a total size of 10", albeit i have found somewhere between 7-9" more commom. Females, like alot of fish, attain a smaller size than the males, with female Jacks averageing 6-7", maybe more but i've never seen a large female of such size. They are far less aggresive than the stigma they are given, as back in the day when they were described by science there weren't such fish as Red Devils, and Green Terrors about, they hadn't been found. They are far more timid IME, and can actually be bullied by smaller cichlids. Don't get me wrong there are Jacks that are as pugnacious as foretold, but in the same way that we have those who are bullied, and those who bully. At the end of the day they are a rather large cichlid, and can more than defend themselves when required, and equally just throw their weight about in general, so care should be taken. They are a very rewarding cichlid to keep, like most, and are fascinating to watch in any case. I would recommend you get a pair, or get a group of about 5 and grow them on. When a pair form sell on the others and have a go at breeding them
Good fun!
If you struggle to get hold of fish in your area there are plenty of business' that deliver to your door. I'm sure the US market is similar to that of the UK in that respect. If that scares you somewhat you can always enquire at what LFS' you do have, you may be pleasantly surprised what they can do for you.
James
