Hi there! We got our boy Mr. Spike 6 days ago. He's almost 4 months old and we're in love! I bought a 20 gallon tank as the pet shop worker said that was fine to start with. But now that I'm researching more, I'm wondering how long this size will even last. So my question is, when should I realistically plan to upgrade? And to what size tank? I just want to make sure he's got the best life possible. Thanks!