I had my adult 8" leucistic in a 10 gallon tank for a while and he just looked cramped in there. If ever something spooked him he would swim right into the decor and injure himself. He likes finding places to hide, so keeping him stimulated without giving him obstacles to injure himself on was a challenge.
Next I tried two 8" adults in a 20 gallon. They looked cramped too, and it was hard to keep them away from each other during feeding. And Falkor found a way to injure himself again.
Now I have them both in a 40 gallon and it's such a relief. They really seem so much more comfortable, and no more booboos have occured.