Everything in your list, except the seahorse, will appear in all 3 tanks after you purchase them.

The plants do what they say they will do, and they are all useful. Are they worth it? Every player has to decide that for themselves.

You can put the ornaments in any tank, giving you an opportunity to decorate your tanks to suit your taste. Their primary purpose is to attract more customers to your store (thus allowing you to sell fish more quickly), and they seem to do that regardless of which tank they are in.

Most people breed Goldbulbs early in the game, and it's a very effective way to increase your money until you get to level 3 of Fish Environment. I have found better fish, though!