Hi! I have a big problem in a tank that I have - it is 45 litres and there I have 2 grown mollies, and around 6 of their babies + 3 snails. I have a biOrb that has cycled already with the nitrite and ammonia levels going down to almost trace levels. I have been doing 30% water change weekly. Recently, nitrite levels went to as high as my test kit showed. After which I have cleaned the tank and changed filter (which is not the biological filter in the biOrb)
Changing 50% water. This helped for a day, but next day you got really high reading of nitrite again. Since, I did 20%water change every day but the readings are still as high...

I am really confused about what I am doing wrong, PLEASE HELP!!

You did not mention bacteria additives, add them in lew of all the water changes. I like "Microbe Lift PL", it smells like sulfer for a bit. Also, place some ammo-chips directly in the tank in a bag or in the filter to remove ammonia and nitrites, only if you DON'T have SALT in you water. Salt will release all the ammonia in the ammo chips back in to the water.

If your water parameters are off, PH and GH/KH are LOW, the bacteria will not flourish. Test for GH/KH, (General hardness and Carbonate Hardness) Mollies like it around 100-200 PPM.
Also, add live plants to take up the nitrates produced by nitrites being processed by bacteria.

I have to ask. Did you use ammo-lock or a similar product, as your tests will be distorted for a while?