I recently had a very similar situation.
I moved my 55 gallon tank into a 180 after cycling etc. Waited a couple of months for things to stabilize and then added some new fish.
All my parameters were good but no new fish would live. all the original fish were fine but new guys didn't make it. some would die right away and some would last a few days to a week and would be eating - then the next day they were gone.
I really don't know exactly what was wrong, but here's what I did and I have recently been successful in adding new fish that have survived for a few weeks now.
First I started dealing with any problems that were obvious. I had green hair algae - although my parameters were fine I obviously had a nutrient problem that was hidden by the algea eating the nutrients. I lowered my phosphates and did a bunch of manual pruning and reduced my lighting cycles until it was all gone. I then fine tuned my temp and salinity. My temp had been climbing a bit - closer to 80 so I increased my chilling and dropped it down to 78. I also found my salt creeping towards 1.025-1.026 so dropped that down to 1.024.
I increased my water changes to weekly and did not add anything new for 6-8 weeks.
I started using the Brighwell bacteria weekly and found that everything in the tank really responded well to that.
I did a really good cleaning of all my equipment and cleaned any detrious from the sump etc.
doesn't really sound like much but I just worked on getting my parameters as close to perfect as I could and ensured that all my husbandry was top notch.
I think the biggest help was just not adding anything for a couple of months - letting whatever was going on in the tank settle down and go away.
good luck.
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