Depending on the look you are trying to get, at least 2 inches upfront should be good for any planted tank. But, when aquascaping, I highly suggest having around 2-3 inches deep in the foreground. And anywhere from 3-4 inches in the background or sloped towards a corner. The reason for the raised ground towards the back of the tank is, to create an illusion of depth among the scape of your tank.
Illusion
By adding more substrate towards the back of the tank, it appears that anything towards the back of the tank is larger than what it actually is. This is because, the eye will perceive it as being further away.
Example
Take this image for example:
It draws the eye towards the back of the frame (think of sloping your substrate towards a back corner) thus alerting our brains that the tree’s towards the back are just as large as the trees closer to the frame. In context of an aquascape, when placing “background” plants towards the back and on a slope, we create the illusion of depth.
Plant health
Consider the Walstad method. She reports that 1 inch of soil and 1 inch of gravel is the most optimal depth for her tanks. This can be applied to all aquatic plants. You may be thinking to yourself, “What about the heavy rooters?”. Fear not, even those will thrive. They will eventually realize that there is no more room to grow vertically so they will begin to grow horizontally.
Conclusion
No matter the plant, no matter the tank size, have a substrate layer that is at least 2 inches deep. This will allow for your rooting plants to sufficiently grow. Also, when planning out your scape, try to create the illusion of depth by increasing the depth of the substrate towards the back of your tank. Until next time, keep your thumbs aquatic!
