Did someone say moon jellyfish?
I'm the owner of that thread that Mark just linked to. My scenario is quite different because the tank is not a tried and true solution like the all-in-ones on the market are. Most of my losses have come as a result of a work-in-progress / trial and error tank. And when I say losses, i'm out $600 in buying jellyfish with nothing to show for it now except knowledge. First 3 died from poor filtration and nitrate and ammonia spikes and not feeding enough. Second 3 died from electrocution. Third 5 died from being too small to begin with and they got caught in the return mesh. I've actually got 2 tiny ones left that I hope are growing their tentacles back. I ripped out the old spray bar and built a new one and in the last 5 days, the two jellies haven't got caught once so I'm hoping my worries are over but the 2 I have left may not ever recover because they can't feed. However, I just don't know if I can really pay $800 for a grand total of 3 healthy jellies if I were to keep this up. I'm seriously considering just trying to recoup some of my investment with all the modifications I've made to the tank and stand and just give up. It's a shame because I think I was finally getting the hang of it.
I've probably done more research on jellies than anyone else lately and i've had several discussions with Wyatt and Nancy. I've learned that it is much better to grow your own brine and vary their diet with other foods I've got from Brine Shrimp Direct. With smaller tanks, you'll want to do a lot of water changes and just keep it clean, which shouldn't be a problem with only a 6 gallon tank. HOwever, with a tank that small, just a small water change is likely to cause huge parameter swings.
I've ordered from both PBN Jellies and MoonJellyfish.com and I think Wyatt's Mediums are typically larger. i would never order the small ones again. At least, not in my tank. They are way too fragile and can't recover from anything if things start to go south.