18 inches front to backYeah nice fish Hoplos are great! Looks to be one of the larger species, like you say gets to about 6 inches but sometimes larger, upto about 8. A four foot is great for them so long as its not really shallow front to back.
Wills
Yeah mate just the one he was given to me from a fish keeping buddyhttp/www.fishforums.net/index.php?/topic/41576-hoplo-catfish/
seems that 4ft would be fine...have you only got the one? says here either singularly or in a group of +3 is best. i saw on youtube someone had a hoplo only setup which was awesome...going to try and find the link for you...