if your cycling then you should be testing for the nitrates and ammonia levels.
And this will indercate when you tank is matured as the nitrites levels will rise and ammonia will be 0 as well as nitrites be 0.
This process take about 4 weeks, if u added fish befor this period the no wander they have died, the ammonia is lethal at low levels to fish.
Once u have tested you water then post the resualts, and some from a few days befor hand, this will help to indercate whats happening.