High levels of nitrite can cause appetite suppression and will eventually kill your fish what I would do is get some 5-1 test strips and test daily and do daily 50% water changes and add a double dose of prime to help detoxify any nitrites till they constantly stay at zero and I would add a small amount of aquarium salt 1 table for 5 gallons that will help prevent mathemoglobin toxicity by blocking nitrite adsorption though the Gills of your fish with out getting your nitrites under control your not going to see any improvement in your Betta @Denied Taco