You can, but you can't add lots of livestock without adding more rock and letting it get covered with bacteria before adding said livestock. In other words, let's say hypothetically, you put your tank in the bigger tank. Now you add 2 five pound pieces of dry rock. Wait a month, then add a fish....add 10 more pounds of rock, then add another fish. I'm not saying 10 pounds per fish is an actual measurement, but i'm just giving a general example that whatever bio load you have, will need that much filtration to sustain it.