I have a 10gal that I use for mine. It is empty until I need it. When I need to use it, I speed up the cycle with dirty filter floss from one of my other tanks. It has a darkened bottom, but I use no substrate for easier cleanup. I have one cave-like decoration and some plastic plants in case the fish need somewhere to hide while they acclimate.
Freedom is the sure possession of those alone who have the courage to defend it. -Pericles
Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and the success of Liberty -JFK