Hey, I think you might be kidding around. If you are, joke's on me. If not, there's a lot of stuff you're missing here that you should learn about before even thinking about buying a fish. First, lionfish (even "miniature" ones, although you're probably just seeing a baby one) grow to be too large for a 20 gallon tank. Second, if you decided it was only temporary, your LFS might not take it back, and transporting him could stress him to the point he'd die anyway. Third, baby lionfish are very delicate; we can't breed them in captivity yet, so they're wild caught, and not accustomed to life in aquaria. That being said, your tank wouldn't be cycled (a way of developing the water quality to its best biological state) if you just threw "some salt and a filter" on it- almost no lionfish alive would survive something like that. Like I said, I won't write a novel for you because I think you're screwing around. But if you're new and just confused, that's cool but please read a bit and feel free to email if you have questions.