Slow down AAmaster. To keep your ammonia and nitrite down you want to have a cycled tank And nitrates want to be removed by water changes. If your tank isnt cycled you want to keep the ammonia and nitrite low with water changes also.
Is your tank cycled?
Before suggesting ph changers id suggest ro
water, most fish will adjust to a ph though and ph uppers/downers can wreak havoc causing swinging or changing ph and this is much worse.
Well anyway what is your ph as it stands?
When you say you cant keep them at the correct levels what do you mean?