If it's constipation , you can try treating with Epsom Salt..usually it's 1 tbsp per 10 gallons. HOWEVER..before you do any such treatment with Epsom salt , know that not all fish can tolerate that, so it's important to research any other fish you have in that tank for what they can & can't have for tank treatments.
When your LFS has a sale , it might be a good idea to get a tank, cycle it, and keep it for quarantine or hospital.