Hello and welcome!

Yes, it would be considered a closed system. You need to check directly with your local governing organizations to determine the legality of the fish you wish to use. Heed badflash's advice and get the details from the local authorities and verify what they tell you. Also, with a small enough system you can always try to pass certain restricted fish species off as pets, but that may not work for your area. In Florida you can pass off certain african cichlids as pets... that is what I heard from a friend anyway! LOL