Betta ammonia problems in tank

Question
I have a betta fish in a half gallon tank. The tank is not made of glass but durable plastic from the pet store. My problem is that the ammonia alert I put in the tank will not stay at safe levels for more than one day...if that.  I have no gravel just a plant --a real one-- since I thought the plastic plant was the toxic problem.  I clean all the food he doesn't eat and his poop, right away. I also use only arrowhead bottled spring water. I do one partial water change every couple of days and also a complete water change every eight days. I do use ammonia block as well when the ammonia level starts reading an alert. My fish is very happy and he eats well. But what am I doing wrong?. What am I overlooking? Do I need to do only full water changes.? Thank you.

Answer
Hello,
A betta should never be kept in a 1/2 gallon tank.  I know that vendors say bettas do well in small bowls, but they don't.  They live unhappy miserable lives.  I am not blaming you, but the vendors who say anything to sell.   The betta is a money making machine.  All the gadgets they sell for this poor fish, is really heartbreaking...not to mention the pills he will need further down the road.  Unfortunately there are no laws to defend fish.  The betta is an intelligent little fish, and should not be treated this way.
A betta should be kept in a 5 gallon tank, or more.  In his habitat the water may be shallow, but they have lots of room to swim.  He is a tropical fish, and needs a heated tank.  His water should be around 80 degrees.  Bettas kept in cool waters will get sick, and their lives will be cut short.
When heating tanks for bettas be aware that this should be done very very slowly.  You put the betta in with the same temperature he is in now, and bring the temperature up one degree every two days.  Bringing up the temperature too fast would be deadly.
If a Betta is kept in a 5 gallon tank with no filter, all the water should be changed once a week.  If the betta has a filter 25% of the water should be changed once a week.  Always use a good water conditioner such as stress coat or aquasafe.
Bottle water is not recommended for fish as they lack the good minerals that fish need to be healthy.  Putting him back on tap water would be great, but you must do this slowly.  Change all of his water, add 3/4 bottled water, and 1/4 tap water.  Wait two days, change all his water again putting 1/2 bottle water, and 1/2 tap water, wait another 2 days, change all his water again put 1/4 bottle water, and 3/4 tap water, and wait another two days, change all his water to tap water.  Remember to use a good water conditioner.
If you intend on keeping your little friend in a 1/2 gallon tank, you will have to change his water twice a week, sometime three times.  This is very important.  Change all of it, not part of it, all the water must be changed.
Chemicals to keep ammonia out of the water do not work, only water changes can do this.  It is very hard to keep ammonia out of a 1/2 gallon tank, almost impossible.  
I would also like to mention that bettas like to have a little toy that they can go through, and you can find these anywhere.  They also like a silk plant to rest on.  It would be a good idea to put gravel at the bottom of your bowl.  It doesn't have to be much, just so that he feels more secure.
Also, vary his diet, and make sure you don't overfeed him.  Betta Pellets, Betta Flakes, mealworms, flaked plankton, glass worms, and bloodworms are very good for your betta, but you must vary.  One day give him pellets, the next flakes, and so on.  Feed him twice a day...example 2 pellets for one meal.  
Be sure to feed him a cooked frozen pea with the first layer removed once a week.  Cut the pea into small pieces, and feed it to your betta one piece at a time making sure he is eating it, and that it doesn't fall to the bottom of the tank.  On the day you feed him the pea, feed him nothing else.  This is so he can clean out his system, and prevent Swim Bladder Disease which Bettas are prone to.
We take fish out of their homes to please us, we must make sure that we give them a home they will be happy in.
I hope this helps, and that you understand the Betta a little more.  If you have any other question, please feel free to write me anytime.
Lynda