nitrite spike in established aquarium

Question
My 20 gallon aquarium (fully cycled for 7+ months) has 3 young angelfish (about 1 to 1 1/2" bodies), one 2" botia, and a gold snail.  It also has a driftwood and is moderately planted.  I do about a 10 to 20% water change and perform water chemistry tests weekly.  I introduced the third little angel 3 days ago.  Each of my fish have been quarantined for 3-4 weeks before introduction to the tank, both to assure their health and to allow the aquarium chemistry to adapt to the increased bioload. I haven't ever had any water chemistry problems until the morning after I added the third baby angel, when testing showed .25 ammonia and .25 nitrite.  Nitrate is at 10, pH is 7.0, and temperature is 82 F.  I did a 20% water change and stopped feeding.  What should I do further?  Ammonia is now better, according to the color chart, but still not zero.  I don't know specifically what the ammonia content is, as my test kit doesn't have a reading between zero and .25.  The color is lighter than .25, but not as yellow as zero.  Nitrite continues to be .25.  How often can I safely do partial water changes and what percentage of tank water should be changed to reduce the nitrites?  Is there anything further I should do to protect my fish and correct my aquarium chemistry?  All my fish still look healthy and they are clearly hungry.  I'm sure I overfed during the couple of weeks prior to introduction of the third angelfish.  The second one I placed in the tank wasn't getting much food, as the one already in the tank was a little bigger, a little faster, and a lot more food aggressive.  I scattered more food over the water surface in an attempt to give the smaller one an opportunity to adequately feed.  I tried to clean up well after feedings, but obviously I missed some food waste and fish waste.  I did cut back on feeding a bit prior to introduction of the third fish, but apparently it wasn't enough!  I accept that I probably created the problem and now I really want to correct it, but don't know what's the best thing to do.

Answer
JoAnn,

Sorry it has taken a bit but I just saw your question. Sounds like you are doing everything right but you might want to look into getting a bigger tank. The angels are going to out grow it very soon. The overfeed most likely added to the problem. As long as when you do the water changes, you recondition the water, you can get away with it everyday. I would do 20% water change for 5 days and then test the water again and see if that has helped. I wouldn't, however, add anymore fish to that tank due to its size. I have friends that have angels and they say  for 3 of them you need at a minimum of a 30 gallon tank, but you now have 4. I would suggest getting a larger filter as well. That might help you out too.

If you have any other question, please ask. I check for questions many times a day and try to get back within 24 hours, most of the time sooner.