starting out with a lionfish

"All right... a complete newbie here looking for advice."

Welcome aboard! Grab some Kool-Aid and let's get to salting that tank =)

"My two year old wants to have a lionfish, and I, of course, know nothing about aquarium (FW or SW). "

While I try to encourage any and all to give sw a chance, placating a child's whim of the week is not a very good reason. Do you foresee yourself being interested too? If not, I'd scrap the entire notion and get the kid a puppy. SW fish can live 10-15 years if properly cared for. Not a responsibility to take lightly.

"I have read a few tutorials and threads here and there, so I have a basic idea on how to get started."

Go purchase these books before you get too far involved. They will prove to be invaluable:

The Conscientious Marine Aquarist: A Commonsense Handbook for Successful Saltwater Hobbyists

and

Natural Reef Aquariums: Simplified Approaches to Creating Living Saltwater Microcosms


"I am planning to get a 50g tank, and my questions are:

What other fish are compatible with (dwarf) lionfish?"

Anything the lion can fit in its mouth is a potential meal.

"How many fish (including the lionfish) would be ideal for a 50g tank?"

Depends on the fish really. A 50g (do you mean a 55g?) is 48" long, so that rules out any and all Tangs. Clowfish are out as the Lion would munch them pretty quickly. Same with Damsels. A hawkfish would work, as would a Fox Face. I'm having a duh moment here and not much is coming to mind I'm afraid.

"I know that whether to have a fish only tank or not is up to me, but does the choice of lionfish influence this decision?"

You will be fish only because the Lion will pretty much destroy any kind of non-sessile (moving) inverts like shrimp or crabs. You can do feather dusters though, they're kinda neat.

Try to avoid relying on hard and fast rules for compatibility as temperments and sizes vary quite a bit with marine fish. One thing that I should point out right now is that dwarf lions are pretty sedentary - most will park themselves on a ledge or overhang during the day and only get active after lights-out. Unfortunately, the bigger, more active lion species get way too big for a 50g tank. Your options for livestock also open up quite a bit if you can make do without the lion...its predatory nature is going to be a real pain when you go to add tankmates.

In addition to the books that Corax mentioned, check out "Marine Fishes" by Scott Michael and take a look at www.wetwebmedia.com --- that should definitely get you started with plenty of reading material!

If you're at all handy, building your own stand will be much cheaper, and allow you to customize the stand for the long term needs (ie, a sump, pumps, equipment storage). Otherwise, I've seen stands at Walmart and most fish stores--your choice of the wrought iron or wood. Often, the box pet stores (ie, Pet Smart) will run sales on the stands.

I'm disappointed in your Wal-Mart...most sell stands and hoods for 55g if they have 55g aquaria for sale. I'd go to a PetSmart, PetCo, or Petland, as I'm not handy, but, if you are, DIY people can tell you how to make one.

For cycling you should not add the live rock. Set up the aquarium as though you were going to add fish (sand, powerheads, saltwater, heater, and all), but leave out the LR. Toss in the shrimp, and wait for your ammonia and nitrites to come down to zero. Then you can add the live rock. The reason for this is that a large ammonia spike (like the one you will get with the shrimp) will kill any organisms on the LR other than nirtifying bacteria. Some poeple don't care to have any hitchhikers on their LR, I think it adds to the diversity of your tank.

Once you are cycled, add the live rock and give it a day or two to settle in and make sure that your ammonia and nitrites are still zero. Depending on whether or not your LR is "cured," it may produce an additional ammonia spike if it experiences any die back once placed in your tank. Once your levels are consistantly stable, add fish and enjoy!

PS- be sure to stock the tank slowly, add the least aggresive fish first, and the most aggressive fish last. Quarantine tanks help a lot, too. The last thing you want is to add a fish with Ich to the tank you have spent so much time and effort preparing.