For long term health and stability with one or two fish get the smallest tank they sell which has light and filter. Good water movement is needed to prevent stagnant areas even in small tanks. If you have lots of plants, you probably do not need a cover but if not, get one with a cover. There are lots of tanks available at the stores of this size. Most are sold as betta or shrimp tanks. But you can put platys in them. Platys need a higher pH like above 7.5 to 8.??, so you can use a substrate suitable for cichlids which helps maintain a higher pH.