First of all, I didn't try my recommendations to be absolute because they are, well... recommendations. If you feel that way then your reply has the same exact tone to it. Dynamictank asked for an advice about buying a macro lens to shoot his 180g tank and I gave him/her one. A 180g tank is gonna be at least 20" deep (unless he has an unusual shaped aquarium, then correct me if I am wrong) and so he/she is gonna have much more use out of a 90 or 100mm macro lens than a 40mm. Comparing a cropped image details to a true 1:1 magnification doesn't make any sense to me either, if you feel like it can give you the same amount of detail then sorry, but my opinion is you've never used a macro lens before or never compared the two.
Coming back to the original topic, I would advice dynamictank to try and see how a macro lens works before buying one, it a serious money but well worth it if you are willing to practice with it and understand macro lens limitations. A good cheap alternative to a macro lens would be a Nikon 50mm f/1.8 lens that sells for around $150. It won't give you the same results as a true macro, but it has excellent optics and it's a great value for the money. Just FIY, I used a Nikon D90 with a Tamron 90mm macro for some of my macro shots, now using D7000 with the same lens.