I see nothing that leads me to believe it is something you did ... Alot of times they are caught with cyanide ...The cyanide causes havoc on their digestion system they eat but can not draw nutrition from the food .... and starve, By the time we get them it is so advanced ...there is nothing you can do for them ... Green clown gobies and all the other ones such as the yellow are often caught with cyanide ... and are a easy fish to care for if you are lucky enough to get one to survive the posioning ... Perhaps take a swing at getting one from live aquaria 2 week guarentee as well as caring for thier stock enough to not allow cycanide caught fish ...
It seems that every one that I had died a painful death when I got them I gave up ..
IF you really like them you can set up a 2.5 gallon QT tank to insure they are fine from capture ... Soak brine in selcon or something similar and add some fresh crushed garlic to trigger the feeding ... Then slowly start to mix mysis with it after you know they are eating ....all my fish were "raised" on brine at the LFS
but took to mysis with in a day .... I also do not feed every day either so it is eat or starve :p in my tank ....Your PH is a bit low but not enough so to cause it in my opinion .. Try opening your top and running a fan across it ,opening the windows will also help to raise it up ... The ammonia you say was after the death , just allow it to cycle through and do a water change if you have live stock do try to do some smaller, H20 changes to combat it , also change your filter midia if you are running it .
Good luck finding one of these lil cuties , but rest assured it is probably nothing you did !!