Not because they're small bags, it's because the grain size is half the size of the normal soil. And this is useful why?
When you place rocks or wood in your tank you want the soil to look as natural as possible right? Right. But the normal sized Flora-Base, whilst being excellent is not the most subtle - by that I mean your eye can be drawn to where the rock or wood meets the soil. Now if you use the 2.5L bag of Flora-Base, the soil grain size is much smaller - less than half actually. So here's what you need to do.
Create your aquascape with your wood and rock using the 5L or 10L bags of Flora-Base. It doesn't matter whether you use brown or black - they both work the same. Take your time - it's meant to be fun remember :)
Then, use a 2.5L bag of Flora-Base and pour this where the wood or rock meets the substrate and you'll suddenly realise why it looks so good. Suddenly the transition is super smooth and it looks very natural. The smaller grain smooths over any cracks and it looks just as nature intended.
Of course, you can also use the 2.5L bag for creating a foreground that looks smooth and in nano tanks where the grain size needs to be smaller due to the overall aquascape being smaller.
