I love this fish and have one myself..However down the road when he gets his mature SIZE I will be trading him back. I saw a full grown one this past weekend, and UH he was slightly smaller than my hand.
NOW: About the DIY in a 55. I think you're going to run into consistancy issue (which lead to BBA) unless you stay on top of the changing out of reactors. When I ran a diy co2 on my 55 I ran 2, cooked in 3 day intervals. This helped until I got lazy and tired of the frequent changing.
OH YEA the tank looks Fantastic..and your ghost Knife would like a tube like structure.