super glue
Believe it or not a certain moderaters son told me to apply superglue to the affected area,and i've been doing it ever since.If it is white band desease this seems to stop it and new growth begins to appear above.
Algae by Julian Sprung
A must read[roscoe]and doesn't cost 50 bucks.But it looks like algae that is a result of new tank,old bulbs,or lack of water changes.Improve conditions and should go away.
Probably opposite
A better choice would be a hammer coral,as it will be more likely to addapt to the toxisity of the rest of your livestock,in such a small tank.place it underneath a power head so flo is over it not at it,and feed well.
Reef eel
Get an engineer gobie you'll never no the differance,and the eel can stay in the ocean where his predatory needs can be met.Only get's 1/5 the size of an ell,and won't eat your shrimp,fish,etc.
bigger sump(refugium)
Unless space issues is the reason for the size,try to put in a spot that will allow a sump of 30 gallons or more to be hidden underneath.I've done a 24 gallon cube this way. It will be a sort of illusion,because you will be able to have a unbelievable amount of live stock...