I feel your pain!! I've done everything! And still all getting dead fish!! I have 85L/ tank, no coral, 6kgs of well cured live rock, tank cycled for 2 months, all water parameters hood, amm0, nitrate5-10, nitrite 0 salinity good, pH 8.3, added 1 chromis, 1 yellow tail blue damsel, they hid in tank didn't take food a week later dead, checked water, ammonia spike(small), left tank, to cycle again for another month, all good again, added 1 chromis, 3 days later it was sick, put it in 20L tank I had trialed before bigger one, it recovered, put it in big tank, 24 hrs dead, half water change, cycled again, for a month, added 1 damsel, water parameters all good, 3days dead! Water parameters still spot on! I have no ideas