their are 2 reasons you could have ammonia, either your tank is not cycled yet or its being overloaded and you need to do more water changes. you need to test other stuff such as nitrite and nitrate for me to be absouletly sure. but as a quick fix do more water changes or try ammo-lock it converts ammonia to a less toxic form.
Nitrofish is right, you either have a new tank which hasn't been cycled, or you just need to change some water.
how old is your tank? - if it is new it is most probably cycling.
if it is older, you just need to do water changes, I would do 30% every 3 days until the problem has gone.
A 10 gallon tank is simply too small for keeping a number of piranha's. They're messy fish (messy eaters, lots of waste), and a 10g simply can't cope with the bioload. Besides that, it's too crowded, even for baby reds: I had 7 in a 10g when I just started: within a month only one remained.
Just my 0,02...
btw: How many reds do you have in that tank, and what size?
Unless you have a baby rhom in it, a 10 gallon is way to small for any p especially in numbers. I can see one being used for a hosital tank but nothing else. Also the smaller the tank the easier it is for water parameters to fluctuate at a dangerous level.