Just basing it on experience. I had them in a 40 long , a 120cm tank, and they were great in there. I'd never keep that fish, or any adult fish, in a 30cm.I tend to disagree. They are shallow water fish that like small spaces and shallow tanks. In larger tanks you never see them and they are scared. Plop them in a smaller tank and suddenly you have a stunning fish that i just wrote. But above 30 cm And it is not it
It's a difference in what you want to see. I'm sure your set up works for you. I'm certain the streams they come from are bigger than 30cm, and they can easily adjust to larger tanks.
Either way - it's a great fish.