First of all, welcome to the NSTG! Secondly, there is no such thing as a stupid question here; everyone deserves to be answered and paid attention to.
So, to answer your question; let's see. If your fish is already in your inventory, you can just head straight to your
aquarium. Hit "Aquarium Inventory" at your top left, and you'll see a little menu come up below to let you see your fish, decorations, backgrounds, etc. Click "Fish," and you can just drag + drop your fish into their habitat. The drag + drop thing also applies to any other decorative items you may want to put in your tank.
Hope this helps!
Ebony the Peacian Vampire
Newbie Supporting & Training Guild Crew
Donations & Charities Subforum Head Moderator
Secondary Moderator of Event Management