Sorry about your Hawk.Unexplained deaths are a reality at times.I can't begin to tell ya the losses over the years,the "leapers" on the carpet really take the cake( can anyone relate?)We just keep pluggin' away.
You mentioned he wasn't acting right from the start,so take some comfort in the fact that you really didn't do anything wrong.
On the bright side,your
PH is kickin'!!Haven't seen a solid 8.3 myself in a long time.
p.s Watch out for certain Hawkfish species.Some consider gobies lunch.
All the best,
Larry.