Hey man, welcome to PFury
I haven't heard of people using grass (assuming you mean the stuff people play football on....
) in their fish tanks, but there are a number of plants that over time form a lawn-like cover across the gravel.
Unfortunately I forgot the names of those plants, but I'll move this topic to the right forum, so our plant experts can have a look and answer your questions.
So:
*
Moved to Aquatic Plants Forum*