I cycled my tank for 8 weeks, put in fish and now my nitrates are 160. I'm using the API masters kit. I bought fish 3 days ago and half of them are dead. I only had time to change about 4 gallons of water out of my 29 gallon tank before work this morning. I will test when I get home to see where my nitrates are. How much water should I change? When can I replace the fish?