thanks all for the replies, theres only been 1 death (scotts fairy wrasse) but got it out about 30 seconds after it died so that wouldn't of caused ammonia I wouldn't think. Havent cleaned the sump for a long time and the skimmer/refugium etc is working perfectly. I did a 25% water change about a week ago doing my usual monthly/6 weekly water change. I use tap water with to top up/water change with the De-chlorinating chemical that ive always used, never use an RO
All the other fish are living healthy lives and no other fish missing. I use API Ammonia testing kit, its probably about 3 months old.
Can't seem to work it out, all I could think is that maybe the tap water has ammonia in it, but where I live the regulations of tap water has to be 0 ammonia.
But Evo8 i'll try and watch it daily and hope fully it'll go away.