Dom, your tank is fairly new -what 3-4 months ? Fishytime was looking for that detail because that's a big factor. Established tanks have different/more stable chemistry.
There is often no "one thing" that causes tank problems; it is most often a combination of factors so being painfully specific when asking for help is crucial -and even then, unless someone has seen your tank recently, you should always take advice (especially if it comes from ME
) with a grain of (reef) salt since they don't have the 'feel' for your tank that, really, only the tank owner can have. Of course, the problem is that as noobs, our 'feel' hasn't yet fully developed.
This will come in time. It's frustrating though.