Photos?

Unless you want a random and purposeless concrete slab in your garden, the best suggestion is to have it removed. You can probably do it yourself with some decent tools and a modest amount of effort :-) Or hire it done if your effort is limited to just signing a check!

I had a concrete fish pond at my new to me house that I removed last year. The backhoe that was here doing the septic and garage foundation work couldn’t budge it. They had to jackhammer it to crack it first, then they were able to pry it out in pieces. Yours is probably not so well built. I wouldn’t fill it with concrete.

If you made a bench seat out of it, it could be filled with fill of some sort -- not solid concrete -- and then capped off. But you'd need to seal it well so water did not get into and accumulate in the cavity. If you didn't need a bench seat, it would be better to demolish and get rid of it and get your space back, finishing off the area in stucco as the surroundings are.

Remove it.

I would remove it on my own. Take your time and do a little bit at a time.

It kind of depends on the functionality of the space. If you want to create an entertaining area, then I agree with installing a bench over it. Natural wood would nice against the tile. If you don't plan on using the area for seating, then I would remove it, no sense in spending money on a bench that would never be used.

I know you want to be rid of it but it would be easier to make it into a water garden. It would consist of aquatic plants under the water. The plants provide oxygen. They stay in their container and will sit on the bottom of the fountain. Many years ago a woman I know made a water garden out of an old bathtub. I couldn't believe the water stayed fresh without circulating. There are products containing enzymes to help it get started. It was not a fish pond but she did add a few mosquito fish for the larvae if any occurred. There is a lot of info online and videos on Youtube.

You can always use it for planter box for herbs or other useful vegetables.