Why do you change water every other day,
also almost a half a tank is too much? It can stress out fish. 1/4 tank in a
month is fine or every two weeks, if you have live plants.
High ammonia, means you're over feeding the
fish or too many fish is in the tank.
High ammonia is deadly to any fish. Ammonia
converts to nitrates and Nitrites later, that can kill other fish, very
quick. Guppies die at over 40ppm of Nitrites.
Cut back on feeding. Don't feed them for
2-3 days, than feed only every other a day for week or so, than once a day.
Fish can go 21 days without eating any thing and show up on their health.
They're cold blooded, you know.
I have an automatic feeder ($20), it can be
adjusted to the amount of food the fish receives. Start very low than gradually
adjust for more food, when my tank is in balance, measuring frequently for
ammonia and nitrites.
Julius