Dead Clowns

I had a few questions about my tanks, considering I have lost 2 fish in 2 days.

I have a 48gal tank that currently houses 1 black ocellaris clown, and 2 yellow tail damsels. When setting up my tank I added in the two damsels and then 2 weeks following I added a ocellaris clownfish. It never ate much, never swam around, and just kinda picked his corner. Then 2 weeks following that I added a sea slug and another ocellaris clownfish, he was active, yet also didn't hardly eat. My aquarium that I got everything from said that I could add another clownfish into the mix, so I knew of another aquarium that did have a black ocellaris so i went and got him and added him about 2 weeks following the last edition. They all actually got along okay, besides the occasional chasing. However, yesterday i woke up to a dead clown (the first one) and today i woke up to another dead clown (the second one).

I had my water checked (I have not had the change to buy a pH or testers besides the hydrometer) and they said each time the water was "perfect" and I could add a fish if I wanted to. My temperature is about 79.2 right now, and specific gravity is 1.023 on the dot. I know that the first two clowns were captured from the wild according to my aquarium, and the black clown was "bred in captivity".

I was just wondering what the heck I am doing wrong. I really do not want to lose my black clown to the same issues. What could they have died from (starvation, sickness)? Should I add another black clown for my current one (or wait for it to settle down)?