Help Progressive : Tank wipe out

My tank has been going 3 months and over the last 4 weeks I have been losing fish at a steady rate of one every week. All params are ok Nitrite 0, Nitrate 5, PH 8.3, SG 1023, Amonia 0, temp 26. The symptoms are the as follows; one day the fish will be happy swimming along normally eating normally, then the next day it will hide away, its will lose its colour, it will display rapid breathing, it wont eat and a day later it will be dead. I have seen some small white spots on the fins of some victims and I have been treating this with Myxazin on my 2nd bottle now (cant use copper, got inverts in the tank). At one point I thought it may be velvet and treated the tank with Exodin
So far I have lost a clown fish, a flame angel, and two green chromis and now the dottyback has stopped appearing.
I think there is more than one problem here, I think the myxazin has sorted the white spot but there is something much more serious in the tank.
I am very close to giving in and letting the disease wipe out the tank.... if I could prove which LFS I got it from I would go round and give them hell !!!!!

The tank is 48inch bow front 240 litres, there are now three fish; a bananna wrasse 1.25 inch(including tail), a yellow tailed damsel 1 inch, a rusty angel 1.5 inch, 1 boxer shrimp, 1 fire shrimp and 6 red hermit crabs. All remaining fish have been in the tank for nearly 2 months. The remaining fish are all eating and swimming normally and show no sign of disease, the only odd thing is the water appears cloudy during the day and tends to clear in the evening.

other useful information...........
Very occasionally the fish appear agitated and dart around and flash on the rocks only to return to normal shorty afterwards. The angel spends quite a bit of time with the fire shrimp being cleaned but the other fish never go for cleaning. The chromis which died showed white patches but the others did not, the first symptom is listless swimming then the fish will then hid away and if you catch a glimpse of it, all its colour has faded and it shows very rapid gill movement. death follows extremely quickly usually with in 24 hours.

In terms of aggression, apart from the damsel occassionally making a rush at the
wrasse (who simply sprints out of the way), there hasn't been any or at least none that I have seen. Although some of the fish types in the tank have a reputation for aggression the species were chosen specifically for their unusually peaceful dispositions (Orchid Dottyback and Blue Yellow Tailed Damsel).

Regarding stock levels there have only ever been five small fish in the tank at any time. I must admit I have been stupid and added replacement fish after the first few casualties (I am speculating that it was during this process that the white spot was introduced).

In terms of grounding I am not sure how I would test this, please advise.

Any ideas on this cloudy tank in the day clearing in the evening, I am gong out to get a microscope to try and see whats in the water.

White cloudy, its very odd, right now at 20.00pm the tank appears clear. You can see ornaments in the room several feet from the end of tank when viewing length wise. (its not crystal clear like my cichlid tank, you know the difference).
In the day you can barely see through it length wise, this has only started in the last few days.

Dotty back just appeared at evening feeding, he has some white specks on the left pectral fin and some more near the head, I think its too small for white spot, hopefully this is being handled by the Myxazin. The fish ate some food but its no where near as active as usual, swimming slowly and hesitantly.