I tend to stay out of threads involving high-tech planted systems because it is not my method and I leave it for those with considerably more experience with it to offer advice. But in the last few posts a couple related issues have come up on which I would like to comment, namely filter flow and light.
I don't want to second guess SinCrisis, but by "blackwater" I would suggest he likely is referring to light. The majority of our soft water aquarium fish, what I call forest fish, do not do well under bright light longterm. They occur in shaded waters; in streams that do receive some direct sun they invariably are found in the shadows. There are physiological reasons for this, but suffice it to say that this is how nature made them and they will only be at their best condition if maintained in aquaria that tend to replicate this type of habitat. The effect of bright light on fish is explored here, if anyone is interested in more information:
Water flow is another cause of stress in fish. These same forest fish should never be maintained in tanks where they are forced to battle currents. Quite simply, it wears them out fast. And again it is not natural to their habitats. I have tanks where the length (4 and 5 feet) allows me to have stronger flow at one end, specifically for certain fish; it is no accident that the characins remain in the quieter areas of the tank...given the option, they easily choose their preference. With respect to the plants, these too do not need strong currents. There is botanical evidence out there to show that plants grow better (photosynthesize better) in moderate flow as opposed to strong flow, all else being equal.
Last comment for now on the remark that we see these fish thriving in tanks with bright lighting and currents. How do we know they are thriving? I would suggest they are not. The late Jacques Gery once wrote that when one provides the environment a species requires by nature, that fish will live its full lifespan. All of these issues that the fish is continually having to fight against cause it to burn out faster because it has not evolved for this environment. We cannot make nature suit our purposes, something that with all the environmental trouble facing us on this planet today we should have no trouble acknowledging.