venting

[RuGlu6]

Why add few fish? Just add one and observe. . How about acclimatizing the fish ? With tank running for a year with no bio load and water changes you should be in pretty good shape and ready. But remember to increase the bio load slowly.
I am sure that you just need to cover all the basics first before throwing a towel.
Salinity
Ph
Alkalinity
Temp.
But most important is acclimatization and SLOW bio load increase not 6 fish at once. Just do one, if it will survive get another one in 2-3 weeks. Adding 6! at once is way too much and irresponsible inmo the fish derves to live and the best chance to survive that we can and should provide. they did not need to die...