NOVELTY - An isolated bacterial strain (ammonia oxidizing bacteria (AOB)) (I) that oxidizes ammonia to nitrite, comprising a sequence 96 % similar to an R7clone140 (AOB Type A) (S1), R7clone187 (AOB Type A1) (S2), R3clone5 (AOB Type B) (S3), or R3clone47 (AOB Type C) (S4) polynucleotide sequence, given in specification, is new.    USE - (I) is useful for alleviating or preventing the accumulation of ammonia in a medium (fresh water or seawater aquarium, or comprising waste water) which involves placing (I) in the medium to alleviate accumulation of ammonia in the media. The ammonia is reduced by 30 % over levels which would otherwise be present in the medium. The bacterial strain is placed into the media by means of a rotating biological contactor or by a biofilter. An oligonucleotide (II) probe is useful for detecting and determining the quantity of (I) which comprises a sequence of (S1) - (S4), in a media (aquarium water) including a material such as aquarium gravel, filter sponges, filter floss, or plastic filter media, which involves: (a) isolating total DNA from the material of the media; (b) exposing the DNA to detectably labeled (II) under conditions by which the probe hybridizes to only the 16S rDNA of the bacteria having sequence of (S1)-(S3) or (S4); and (c) detecting and measuring the hybridized probe for detecting and measuring the quantity of the bacteria (all claimed). (I) is also used in a bioremediation process to reduce the level of pollution caused by ammonia. The detection method provides an effective tool for one to monitor and detect the occurrence of bacteria capable of oxidizing ammonia to nitrite in a medium. The method also provides tool for one to check the commercial additives to determine the effectiveness of the additives, particularly in freshwater aquaria, by measuring the occurrence and activity of the bacteria.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a biologically pure culture of (I) where the 16S rDNA of (I) comprises (S1) - (S4);    (2) a composition comprising (I);    (3) a composition comprising concentrated (I), where the 16S rDNA of (I) as a sequence 96 % similar to (S1) - (S4);    (4) an isolated nucleotide sequence comprising (S1) - (S4) or its variant which is 96 % identical to (S1) - (S4); and    (5) an oligonucleotide probe (II) consisting of 5'-CCCCCCTCTTCTGGATAC-3' (S5) and 5'-TCCCCCACTCGAAGATACG-3'.