Big tanks are OK although I found the standard 6" 120/125s to be far less stimulating and useful than tanks that are taller and deeper. Not to say that it's not amusing to have two to three larger species setting up "dens" at opposite ends and maybe also in the middle of a six foot tank simultaneously (big fun when they brood at the same time and the fry start mixing), but generally I'd take a 18" x 24" x 48" 90 gallon any day, and better still a 2' x 2' x 4' 120 (for my money the ultimate configuration). Like 125s, 55s are essentially display tanks - but exhibiting fish is generally different than breeding them and as Dr. Lefler pointed out territory is about acreage much more so than verticality, as is serious internal planting/landscaping which is far easier to accomplish/do well with more ground.
40s are called 40 breeders for good reason, and if you have the space to do racks of them, brilliant/rock on, but if one is balancing appetite with size/budget constraints, hard to do better than racking 10 gallon tanks endwise top and bottom on a 6 foot double stand - perhaps no better, more compact, cost-effective way of spawning a dozen+ different Apisto/dwarf cichlid and/or Corydoras species all at once in a small space (and with only one medium sized air pump and two 48" lights). When I broke down my 45 tank/1000 gallon array in DC, the Eheim canisters and the two old 6" double shelved iron racks we're all I kept (the newer ones are generally ugly and less sturdy). At $50 a pop was the best money I ever spent on the hobby except for maybe Chriscoli's fish-whisperer lessons.
