some people will disagree with me but if you can promise yourself, that you will do 25% water change once a week(every sunday or every saturday or whatever) I guarantee you won't have any water chemistry problems if you put two JD in a 55 gallon. There may be some aggression issues but if you get them both at a young age and let them grow together, I am sure you'll be fine. Although I said yes to 2 JD in a 55 does not mean you can put 1 JD in a 40. You need the 4 foot tank and the 12 inch width, for swimming space and surface area. Buy a 55 gallon, Aqua Clear 110 and weekly water changes and you will have a healthy pair of JD