Little white pests...but not Aiptasia...and other problems
Hi, hope you can help!
<Will do my best>
Background:
We have a 125 gal saltwater tank with 2 filters, protein skimmer, but no reef lighting (yet).
<... but some lighting?>
  Inhabitants are a yellow tang, Firefish, coral beauty, 2 cleaner shrimp, a pink knobby Cuke, a dozen or so hermit crabs, and damsels.  (Neon damsel, blue damsel, humbug damsel, 3 green Chromis.)  We have several large live rocks, and a few pieces of fake decorator coral junk while we wait to get the right lighting to add the real stuff.  Substrate is a mix of live sand and crushed coral.  Chemistry and temp perfect.  Tank is about 6 mo.s established.  All is well in there.
We have a 20 gal hospital tank for inductees and sick guys, currently only inhabited by 2 cleaner shrimp and a few hermits, about 4 months old.  A few pieces of decorator plastic caves, and a few pieces of live rock.  Same substrate.  Same chemistry and temp as 125 gal tank, but our new fish don’t survive it… not sure if something is wrong in there or if we are getting bad stock or what’s going on.  
<Good list of speculations... you are using your big tank's water there? I would>
We don’t want to introduce new fish into the big tank, because we’ve already experienced “chasing down the sick guy” in the big tank, and it’s not good for any of the other inhabitants.
<Well-stated>
  We had two Lunare wrasses in the main tank for about a month that were trying to eat everyone else, so we moved them to the hospital tank for a few weeks before returning them to the pet store, and they were fine in there.
<Interesting... gives weight to the "initial bunk livestock" theory>
We feed with frozen multi-pack foods in the eve, and leave dried seaweed pieces pinned up during the day, both of which are eaten voraciously, but nobody looks to be starving, or even close, and the shrimp are molting and hermits moving to new homes, but no visible food decaying or spike in nitrates, so think we are good with feeding.
Problem #1:  Every time we get new fish, they die in the hospital tank, usually within 3-5 days of arrival.  We do 10% water changes (in both tanks) every week.  We have tried a 100% water change in the hospital (several times).  We have tried restoring it with water from both the main tank and clean water source to help match.
<Ah, I see>
  (Water source is completely purified, we bought a water purification system solely for the tank water, and add Oceanic Salt.)
<I would read over re synthetic salt mix brands... on WWM, the Net... and switch to something else (myself)>
  Occasionally we see signs of ich on the new guys…when we do, we remove the carbon in the filter and treat with Kick Ich….
<This product is worse than worthless. Please read here: http://www.wetwebmedia.com/homeopathfaqs.htm
At best it's a misleading placebo, worse, it may be poisoning your system to a degree>
but sometimes no signs of ich, they just keel over.  We used to induct our fish over a several hour period to the hospital tank, lights off, adding ¼ cup to their bags at a time until introducing, but have gone even farther now to a “drip method” where we place the fish and water in a (covered, dark) container, and slowly drip the hospital tank water into the container until it overflows into a bucket (4-6 hours or so), before introducing the fish into the tank.  Still they die.  Any advice or thoughts???
<Lots... for here, try a PolyFilter in your water flow path... see if you "get any color"... You likely have a poison source here... perhaps a bit of metal from... a clamp? Ornament? Check with other hobbyists in your LFS/source store... do they have similar lack of "luck?">
Problem #2:  Several months ago we were thrilled to see a bunch of “little white things” birthed in our 125 tank and floating around.  (Hey, stuff is living!!)  =)  They were free-swimming, with a tiny ring of tentacle looking things around the top, sort of hydra-looking, and may have been Aiptasia,
<Nah... not this life's M.O.>
but if so did not last long.  (Probably got eaten.)  About a month ago, there was a birthing of same in the hospital tank.  We had added some live rock, and also infused some water from main tank, so don’t know source.  At the time there were (doomed) fish in there, so only one of the birthed critters survived.  He lodged himself on a shell and grew to be about an
inch or two long!  He was white, with a large rotund belly, and a small hole at one end.  
<Sounds like my ex-brother in-law>
No visible tentacles, really, but a small circle of very short hair like things around its hole.  Interior appears almost hollow.  No narrow base, far removed from Aiptasia descriptions, obviously alive due to growth, but not movement, (stationary in all regards), and very ugly.  Sort of like an onion?  After another batch (3) of new fish died, we removed him
as well, as we did not know what he was and he was therefore suspect. (About 3 weeks ago.)
<Likely not related to your fish mortalities, and don't know what this is exactly... see below>
Last week, there was another birthing of these things in the hospital tank!
They appear first as free-floating creatures with little umbrella heads (very tiny), almost like hydras, but then affix themselves everywhere—the glass, heater, shells in the substrate, and commence to filling out with the wide belly onion look.
  They are growing, and are now already probably 1/8-1/4 inch long.  I have searched and searched and can’t determine what these things are.  They fit the description of Aiptasia in how they arrive and behave, but not at all in appearance after that.
<Actually, not an Actinarian (anemone) but likely a hydroid of some sort... a guess based on your description of appearance and life history. Not desirable, but no reason to panic either... they are originating from a bit of live rock (strobilizing is the fancy word). They will "run out of eggs" someday soon.>
Any help?
<Oh yeah, getting to something in the way of a real solution... Here's the big wind-up and the pitch.... a refugium! I'd attach another (live) sump container to your existing system, put it on at least a reverse timed lighting schedule (do get some light for your main tank if you don't have this as well), and grow some live macro-algae, have a DSB there... Voila! Whatever the problems are/were, solved. Please read here: http://www.wetwebmedia.com/refugium.htm
and the linked files above till you understand rubber band. Bob Fenner>
Thanks,
Tracy