If you don't already have one, you might use this 5g as a quarantine or hospital tank. Just set it up, cycle it, and add enough food or straight ammonia daily to keep it cycled. That way, when you buy new fish you can keep them separated for a few weeks or a month to make sure they're healthy enough to add to your main tank. Or, if a fish in your main tank gets sick you can place it in the 5g for treatment and perhaps avoid having to medicate your entire main tank.