Ammonia (NH3) and ammonium (NH4+) are substances which should be converted rapidly into nitrite followed by nitrate, called cycling or nitrification. If this does not happen, then the aquarium is not fully cycled and thus safe for fish. In a well-functioning aquarium the ammonia value should be 0.1 mg/L or lower.  If ammonia values are high then a partial water change should be done. Especially in the initial or start-up phase of the aquarium or pond the filter bacteria are not been developed well and so regular monitoring is very important to avoid ammonia or nitrite toxicity to animals.  Keep out of reach of children.