Described herein are novel ammonia-oxidizing bacteria as well as isolated nucleotide sequences representative of 16S rDNA of these ammonia-oxidizing bacteria. Particular bacteria of the present invention are tolerant of freshwater environments, saltwater environments or both. Furthermore, in various embodiments, various bacteria of the present invention are capable of surviving a freeze-drying process, and may remain viable thereafter. Methods for preventing or alleviating the accumulation of ammonia in aqueous environments, such as aquaria and wastewater are also provided, using the ammonia-oxidizing bacteria of the present invention.