An aquarium - usually in a home - that could not actually support a fish. It follows a cartoonish, picture-esque fishbowl appearance rather than that of an actual proper, healthy aquarium.
The three main characteristics to look for are:

• Too small for the fish: as in only a couple times larger in width than the fish is long. Depends largely on a particular species' range and needs. In exaggerated cases, the fish may not have space to move.
• No filtration: there is no visible filter or enough plants to keep the water safe.
• Overstocking: there are too many fish for the space, making it crowded, uncomfortable and susceptible to disease.
• Lack of stimulation: the tank lacks places to hide and explore for the fish.

It's worth noting that while in real life these are lacking tanks that could be considered animal abuse, art is fictional, and despite real-world problems the simple fishbowl has connotations an artist might like. This does not mean that they should be harassed for this. If it's worth addressing, leave a simple, helpful word in the comments.
Aquariums will also almost always be inadequate for whales, dolphins, and open ocean sharks such as the great_white_shark as a functional man-made aquarium couldn't possibly be large enough to give these creatures the space or range they need to live comfortably.

See also:

• aquarium
• fish bowl
• fish_tank
• pet
• fish
• terrarium
• vivarium