When you say cover the bottom, do you mean substrate or plants?
Substrate - I use a small inert gravel (1 - 3mm) with a mixture of APIs First Layer Pure Laterite in the bottom third layer. This provides minerals for the plants and the small gravel allows easy rooting. I also use a heating cable under the gravel to stop the gravel from going anaerobic.
Plants - Depends on your water parameters i.e. hard/soft, lighting etc. but there are plenty of hardy, easy to grow plants out there. There's loads of info on the net. Try http/www.thekrib.com/Plants/ for starters.