If is new tank then u can follow the below, that the way I did to bring down the ammonia and nitrite to 0 after I overstock my tank with fishes...
Small water changes 20 to 25% for every 1 - 2 days (Never use tap water as the nitrite level is high in tap water)
Dose seachem prime to temporary change the free ammonia NH3 to ionised ammonia NH4, only NH3 will kill fish
Dose prodibio biodigest 1 vital per day, I find it more effective than Microbacter7
Add more bacteria hosting media like, marine pure, maxspect or bio-home
Reduce feeding or cut down the food
Stop adding fish and wait for the bacteria to build up
After doing the above for 3 weeks then I see the result...