CorallineAlgae
Aquarium Advice Addict
Honestly, I never kept more than 3 fish in my 30L and that's with a refugium. If you want another fish a sand sifter would be the direction to go. All of them are cool especially if you can get one paired up with a shrimp. I'd wait at least a couple of months to let your tank age and start to settle down. It honestly takes 6-12 months before you can begin to call it established. Better to get the things you really need to make it great first. Once your tank is rockin' another fish would be ok.