what caused this?

my fish tank as been cycled for over 6 months and i haven,t added any fish in that time all readings where great 0 ammonia,0nitrire,7.0 ph,10 nitrate,
now all of a sudden this week my ammonia as gone off the chart ive added no new fish or had any fish deaths,so what caused this??

anyone,,someone must know why

There is a couple of things that MIGHT have caused this to happen.

1. Untreated tap water - A few months ago I had the same thing happen and although I also use declorinator ..this once I forgot and the tank went into a full cycle

2. Medications - Some can kill bacteria and again put the tank through a cycle

3. Faulty test kit - Might be worth checking if all esle fails.

Hope this helps

you may have old tank syndrome
read about it here
http/www.bestfish.com/oldtank.html