I wouldnt give up just yet,
I know it seems mean and heartless but sometimes fish die with no apparent reason. You know why the first chromi died so no mystery there, and the second one, well chromis just dont last sometimes, it's just how it goes.
As for the clowns, again, stuff happens that you cant actually place the blame on one thing or another.
Just try again, get a small fish, acclimate him well and over a good amount of time. Check your parameters before adding him and after a few days. I think and believe you just caught some bad luck. I'd be embarassed to say how many fish I have had die on me. Some were my fault and some were not.