Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
No problem at all - won't take long.
Catch & place your 2 tetras & pleco temporarily into a bucket filled with some water siphoned from your tank.
Remove your gravel substrate - clean up your tank a bit - siphon out some more of the no doubt cruddy water.
Put your new substrate in place as you wish it. Partially refill with conditioned tap water- re-start your filter, and put your fish back into the tank, with some of the bucket water if need be. Done.