An Accidental Hologram?

If it were a reflected image from the front surface, it would appear to be inside the aquarium. It could possibly be a reflection from the far side of the aquarium, which would be convex from your viewpoint. In that case, the image would be inverted.

"Reflection from Convex Mirror

The image produced by a convex mirror is always virtual, and located behind the mirror. When the object is far away from the mirror the image is upright and located at the focal point. As the object approaches the mirror the image also approaches the mirror and grows until its height equals that of the object."

"Reflection from a Concave Mirror

When the object is far from the mirror, the image is inverted and at the focal point. The image is real light rays actually focus at the image location). As the object moves towards the mirror the image location moves further away from the mirror and the image size grows (but the image is still inverted). When the object is that the focal point, the image is at infinity. As the object moves towards the mirror inside the focal point the image becomes virtual and upright behind the mirror. Initially the virtual image is much larger than the object and really far away, but as the object approaches the mirror the virtual image also approaches the mirror and decreases in size."

https://www.acs.psu.edu/drussell/Demos/RayTrace/Mirrors.html

Another possibility is that the aquarium glass was polished with a cloth that left streaks or scratches. You've probably seen the same thing through windshield wiper streaks. It's called a "scratch hologram".

https://www.youtube.com/watch?v=reufppmXJ0g

I once got an old XY plotter and replaced the pen with a needle point stylus and wrote a program to generate a scratch hologram of a cube on a piece of plastic. It actually worked but is a bit hard to see. The stylus didn't cut far into the plastic.

It sounds sort of like a pepper's ghost effect but with a curved surface making it look like it is in front instead of behind the glass.

The difference between that and a hologram is that it is reflecting a 3d object to begin with and nothing was ever 2d. Same thing with the pepper's ghost like used on the haunted mansion for the dancing ghosts at Disney. The effect look good because they are real just that you are seeing them in a partial reflection.

My understanding is a hologram looks 3d and is usually an image of a real 3d object but is stored on a 2d surface through interference patterns.

If you need to have the real object to create the effect and can't store it somehow then you might as well just look at the real object, unless you are going for a ghost effect.