If you’re new to shellies (typically short for shell dwelling cichlids), give serious consideration to the size tank you choose. Sure, shellies will work in both small and large tanks. However, if you don’t already have a tank, consider one that is lower profile.
If you’re planning a shellie only tank, go for footprint over depth. By footprint I mean the length and width dimensions. The heigth is less important. Why? Shellies are bottom dwellers that mostly get up into the water column only to feed. So focus on giving them a larger floor space.
For a shellie only tank, a standard 55g or 75g, both at ~21 inches high (that will vary a little depending on the manufacturer), are taller than needed. You’ll have a lot of dead space because the shellies will spend the majority of their time in the bottom third. On top of that, you’re using a lot of unnecessary water. Unless you’re on well water, collecting rain water, or relying on source water you don’t pay for, you’re throwing money away when you do water changes.
There are plenty of tanks out there that are perfect for bottom dwelling species like shellies. One example is the 50g Low Boy breeder by Zoo Med (see photo at top). Of course, tanks like this aren’t necessarily meant for fish, so don’t expect to easily find a lid/top for them. A company named Mr. Aqua also sells several low profile tanks. You can find them at SevenPorts. Like the Zoo Med Low Boy, Mr. Aqua tanks don’t come with covers/lids. Some shellie species are jumpers. Thus uncovered, low profile tanks can be risky for a colony of shellies.
Another option is have a tank custom made. There are plenty of custom tank builders out there. However, be aware that a custom tank will likely cost you considerably more, especially since you’ll have to have it delivered to you unless you live close enough to the manufacturer to pick the tank up yourself.
On the other hand, if you’re keeping a mixed or community tank, then consider a size that best accommodates the other fish. Regular tanks by Aqueon, Marineland, and others will also accommodate shellies and other species.