Your mom should get a very very tall tree..

Just kidding.
All the stuff is probably needed, but she could have "santa" wrap them on Christmas Eve (nets, fish food, thermometer, filter if you wanted to clean it or have the wrap be a soaking mess

, etc) and by the time you rush down stairs eager to see what "Santa" left you, everything will be just fine.
As far as making the tree look well populated with presents...you could suggest to her to wrap empty boxes, or wrap a box and inside have a card saying, "Go to <whatever room as the fish stuff> to see your present(s)"
It is hard to mix populated fishtank and traditional under the tree present opening.
