Please be careful when adding new livestock to your tank, especially sea apples, medusa worms, COWFISH AND BOXFISH.
If you add new stuff to your tank and fish go to the bottom, breath heavily and/or die, toxins may have been released from new inhabitants because of stress. Some of these toxins are extremely strong and act very quickly. If your are faced with this dilemma, CHANGE A LOT OF WATER IMMEDIATELY. Also run a small amount of carbon for 2-3 hours flipping the bag to expose all of the carbon to water. Water passing through carbon is much better than passing over it.
Replace carbon with a new batch and change it after 8-12 hours. This should solve the problem but repeat if necessary.
To avoid this problem, acclimate cucumbers, etc. as described on our DRIP LINE PLUS page
Avoid sea apples and medusa worms all together.
The thought that leather corals should all be kept at the end of your tank where the overflow is to prevent them from poisoning other corals is ridiculous if you have any circulation in the tank. Any "toxic" effect will be spread dozens of times faster by internal pumps than it will be removed to your filter below. This also implies you cannot keep leathers unless you have a sump type filter - not true.