new planted freshwater 55g tank. my fish keep dieing

Geomancer

I don't believe so, I've seen it mentioned several times.

Ammonia levels can be higher before they're instantly lethal compared to Nitrite. Both though cause permanent damage that does not heal over time.

Byron

One frequently reads that nitrite is the lesser compared to ammonia, but one has to take that lightly. As Geo said, both at any level above zero do cause damage, and as far as I know, this is permanent--even if the fish survives it will have health problems and almost certainly a shorter lifespan as a result of the ammonia/nitrite.

If the temperature variation is due to weather, there isn't much you can do about that. I've had temporary highs over 90F in past summers and no fish died. Feed less, don't mess with the tanks (this stresses fish and increases their metabolic rate) other than maintaining regular weekly water changes.

I am not one who recommends StressZyme, as it messes with the bacteria and in spite of its claims it will increase ammonia temporarily. A cycling bacteria supplement would be better, something like Tetra's SafeStart or Seachem's Stability. Or if you can get it, Dr. Tim's One and Only. These are live bacteria and they do work to quicken the establishment of the nitrifying bacteria. Live plants, esp fast growing stem plants and particularly floating plants will work wonders too, more than anything else.

Water changes of 50% as others suggested are good to keep the ammonia/nitrite diluted.

Byron.