Flat tank

[dreamsr]

Several life changes and home shifts later, I still haven’t set up my first tank. I’m not going for size here. I want some colorful anemones & soft corals, preferably self-sustaining. Thinking about where I could put one in my apartment, the best spot seems to be the mantle above the (non-functioning) fireplace. It’s about 10” deep. If I could find a tank that would fit and is 2-3’ wide, is there any reason not to use it? Would there be issues with, say, circulation? Any suggestion on where to find a tank with those dimensions?

 

[Fish Fan]

You could search for a tank called a "bookshelf tank", these are more popular for freshwater, and sized roughly to fit a bookshelf (very narrow front to back, but very wide left to right).

That type of tank may work, but typically you'd need some space behind the tank for either a Hang On Back (HOB) filter, or an overflow. Certainly, there's other options, but those other options may not be the easiest if you're newer to reef tanks.

Most in reef tank keeping tend to value the front to back dimension more than even height. The reason is that this gives some real depth to your aquascape. And as you pointed out, getting flow right in a very narrow and wide tank may be challenging.

I'd suggest you look for a small All-In-One (AIO) aquarium, such as those from Innovative Marine, for example. You can find AIO tanks from about 3 gallons up to 40 gallons and even larger. But the smaller ones like 10-25 gallons can often fit on a countertop, desktop or even something like a sturdy end table or night stand.

This 15 gallon AIO would be a fine first tank, if you're short on space. Much smaller and you're really limited, much larger and you may need a dedicate stand and floor space. This particular listing appears to be the old price too, so bonus :)
https://www.saltwateraquarium.com/15-gallon-fusion-aio-all-in-one-cube-aquarium-innovative-marine/
Good luck!