I have had my 36 gallon tank for 3 weeks. My amonia was always at .25, could never get it to 0, but I haven't lost any fish, and they seem fine. I added some more fish a couple of days ago, and they also seem to be doing well. I just checked my amonia (because of adding new fish), and it is now up to .5. What would you recommend? Adding something to the water, or partial water change? All other levels (Ph, etc.) are in the ideal range.