Ok I have a 20L frag tank. It has been setup for around 6 months. Both of the acan echinatas in question have been in the tank the whole time with no problems. I feed them mysis and some gel food around 3 times a week. They have been responding well, each started off with 3 heads and both now have around 10-12 heads each. But just a couple of days ago I noticed they both were deflated and have not come back. I don't notice any missing flesh expect for one or two small spots. I dipped them in tropic marlin pro reef care and moved them away from the light. Everything else in the frag tank (acan lord, chalices, lower end sps, etc) is doing great and growing. Params are spot on minus mg:
dkh 7.7 (dosing mb7 and biofuel)
ca 450
mg 1250
sg 1.025

I have read that mg should be around 1350+ for acans. Anyone else heard this. I know the mg should be higher anyway, so I'll take care of that tonight.

Two possible causes that I can think of:
The most recent change (most likely cause) was adding an aiptasia eating filefish to eat some aiptasia and leftover food. LFS guy said he may nip at my duncans but everything else should be fine. Added the fish and after couple of days the acan trouble started. To solve this problem I created an eggcrate partition to keep the filefish away from the acans. Only problem with this is no other coral is showing any signs of stress/nipping.

Two, is I switch the MH bulb to a 20K EVC from a 20K unknown LFS used box. I switched the bulb a couple of weeks before the acan trouble started, so I don't think this was the issue. But I moved the MH over closer to the SPS side of the tank and move the acans to the far end (partition blocks the light as well).

Any thoughts? Any hope they will recover? I'll grab some pics tonight.

My guess is that it's the fish or the bulb change due to the timing. How long after you added the fish did they start looking bad? It's definitely not magnesium so no worries there.

I think the purple acan started looking bad first, around two days after adding the fish. Then the green one a day after that. Miguel, just got you book last week, great information, thanks! Filefish is headed to the main tank I believe.

Great to hear that you liked it, and thanks again!

Looking forward to seeing if they reinflate once the fish is out, usually those aiptasia eating ones like to sample various things throughout the tank unfortunately...

Yeah the filefish wasn't such a good idea. He loves mysis though so maybe if I keep him full on that once I move him to the other tank. Currently there is no way for him to get to the acans in question since I put the eggcrate partition up. I did that a two days ago and nothing yet. I may try to feed them a little tonight and see if I get any reaction. Sucks cause they had just started with growing like crazy about a month ago. Hopefully some heads will come back.

I have the solution! Post a picture update for your 6 gal biotope and all of your problems will disappear!

Ha, I didn't think anyone looked there anymore. It is still up, just neglected. I started this frag tank around the same time and been working on it mostly. Probably going to take the 6 gal down for the time being. The sole inhabitant, a purple lobster, chills in there by himself. I added a turbo snail a couple of weeks ago and he has been eating some of the macro.

They looked a little better last night. In addition to looking deflated, both acans looked slimy, a little bleached and the mouths were open. Now a tad more inflated and the mouths have closed on all of the heads. The little dimples (coralites?) are noticeable again as well. One of the heads on the green acan seemed to eat a little food last night. So hopefully they will keep recovering.

FYI, they are back. Took them a couple of weeks but now they look better than they did before the filefish. The little SOB got my acan lord as well before I could move him. Chomped it up pretty good. It to is on the road to recovery and almost back to its previous self. I move flats (the name my wife gave to the filefish, I have a different name ) to my display tank and he is behaving much better there. He doesn't seem interested in the corals in there: rics, candy canes, blastos, toadstools, torch and some zoas/palys here or there. I'm also making sure to keep him fat off of pe mysis as well, he love that stuff.