Nine times out of ten this is a water quality issue. This can be caused by ammonia, nitrite or high nitrates. If your test kit is not out dated and still good then according to the water perameters you posted this is not the case. Another reason this can happen is and in this instance the most likly sceniero, Ph. A low Ph around 6 or less can cause this, particulary when a fish is placed in a Ph that is much lower than what it has been kept in, this will cause the eyes to cloud and can cause deth as you have experianced. Over time the fish living in the tank adjust to these conditions as the Ph slowly drops, a sudden change however as with newly introduced fish will shock them as they can not adjust as quickly and you will see the eyes cloud and in some instances depending on how great the differance is death, which you are experiancing.
My suggestion would be to test your tap water so you have a base line from which to work, this way you can track the Ph level as it drops and adjust you water change/maintenance schedule accoringly. You may also need to buffer your Ph by adding some crushed coral to maintane a more sutable level 6.5 to 7.5 is reasonable. When ever you gat another fish check to see the Ph it's being kept in so you know the differance between it and your system then drip acclamate the fish in a bucket over several hours to slowly acclamate the fish before introducing it to the main tank. This will help prevent future losses.
