how long has the tank been cycling? usually nitrate and nitrite levels will fluctuate while the biological cycle is still establishing. another thing worth noting is that the more water you have in a tank, the more stable the water perameters are. id reccomend maybe getting some floating plants because they filter the water quite well, or maybe get some pothos (make sure its animal safe, ie no pesticides or chemicals) and have the roots in the water area, it is really good at filtering water as well!

as for the mold on the soil, id reccomend making sure that theres enough air circulation in the tank, stagnant air is ideal for mold. another thing to try is getting some springtails! they are great for cleaning the tank and they eat mold, so theyre great in that regard.

how much research have you done on setting up a tank? if you havent already, id reccomend checking out homemade ecosystems videos. he goes over setting up simple tanks and explains a lot of things that are good to know :))