Gravel is an important part of any aquarium aquascape. It covers the base of the tank and is like a 3D blank canvas for you to decorate. It gives a supportive base to hold your feature-pieces of rock or wood in position and has the right structure to encourage good root growth for your plants.
Need help choosing?
The quartz gravel is all perfect for growing plants in - the grain size is 1-2mm so water can still move through it for gas and nutrient exchange and roots can grow through it without getting damaged. Chunkier-grade gravel is fine if you aren't growing plants, but you will find it harder to keep clean as waste gets trapped in the large gaps between the grains.
How about using sand?
We would advise you to avoid fine sand as the particles are too close together so it compacts over time, squeezing all the oxygenated water out. This means any bacteria activity under the gravel has to be without oxygen, so you get colonies of anaerobic bacteria, which means when you disturb the sand you get those swampy bubbles belching up through the water - nice! Sand also collects waste on it, which will drift around like tumbleweed and pile up in dead-spots of current, e.g. at the base of plants or rocks. This looks awful and will need regular vacuuming to keep it looking good (and the sand easily gets sucked up the syphon tube and gravel vacuum so you'll end up with a bucket full of sand and a bare-bottomed tank!)
How much do you need?
To get a good enough coverage for healthy plant growth, aim for about 5cm depth at the front of the tank, ramping up to 10cm at the back. Roughly you'll need 5Kg per 30cmx30cm base area. If you really want plants to do well, add the AquaBasis substrate fertiliser under the gravel - now is the time before the water goes in!