do you cut through the mouth?
They will recover faster if you do but it is not needed. If there is a piece of shroom on a rock it will become a colony in time.
The 400 is about $38.00 and the 1200 is about $60.00. I can't give you the site here because it's not allowed. Try your LFS or do a search on the web under Hydor Koralia Pumps and see if that gives you anything.
Mine also got big and then split. Had it for about 10 months when it got to be about 12 inches across, split so I have two sixes. When I got mine it had nice bubbles then lost them after about 2 months. I've had it a little over a year now. I have heard if you feed it alot it will split sooner...
I would just get a regular tank that is 48" long and do what maxalmon said with the egg crate. Build a tier system out of the egg crate so you can keep different corals at different heights for the different light requirments.
twoamp sorry for your loss, if he did pass on though he can put alot of toxins into your tank. I would do a major water change asap and try to locate it to get it out before you have any problems.
IMO you should feed once every other day and only the amount that they will eat in about 4-5 min. . Allthough feeding time is the best time to watch your critters if you feed everyday then only feed the amount that they will eat in 3 min. and feed slowly so not to feed the tank as much.
I would frag. Mushrooms are one of the easiest corals to frag. Go small your first time just to get the feel for it. Maybe just take one of the shrooms off the rock its on now and move it. Once it reattaches then take one more and cut it in half and attach it to another rock. Good luck.