Since your tap water contains no nitrates, then its coming from the bio load in your tank. Other than using a denitrification system, or nitrate removing resins the only way to remove them is through water changes. Live plants will remove nitrates also, but with the type of fish you are keeping that probably won't work! Try the 75% per week WC
's and see if that helps. Even though you have a large tank, the load is pretty heavy, and cichlids produce a fair amount of waste. If once a week WC
's don't produce the results you want, then try twice weekly changes. Eventually you will find the routine that will keep the nitrates under control.