In the new  millennium  and with  the  advent  of high  yielding  varieties of  paddy,  the  use  of various  pesticides,  some  of which  may  be in minute  quantities are  highly toxic  to  aquatic  life.  Fish  culture,  therefore,  is not  more  compatible with paddy  farming wherever the latest  high yielding  varieties of paddy  are cultivated. Champaran (Bihar) is known to be an area of high quality yielding rice  and are even  exported  to several countries. There  are numerous swampy and derelict lentic  water  bodies  in this  area.  In recent years.  however,  attempts are being made in the fish  culture to  utilize the vast swampy and  derelict water area  for immediate use  for  the  benefit  of the  poor,  without getting involved   in to  costly reclamation  process.  Despite  of  poor and  unorganized capture  fishery,  the  percent yield  was  estimated to be good. The fishes like Channa, Heteropneust, Anabus,  Clarius and even some  Carps  are being cultured in the paddy  fields  as well  as in the water  bodies  of this  area. By virtue of their  hardy  nature and air breathing habits in many cases,  they  are excellent  material  not only  for utilizing  swampy and  derelict water bodies but also  in paddy  fields as they  permit  high stocking  density  and respond to supplementary feeding.  Their  production potential is by and large  directly proportional to inputs  and intensity of operational management. Despite of good  inputs   of  these  commercial  fishes,   the  potential toxic   compounds in  various chemicals  used  for agricultural  and other  domestic purposes get  distributed by  a variety of  means  and  accumulate concentrations in the soil  and water.  These  accumulated organic  compounds lower the DO level and does  not support  fish  life.  Fish  species  are far from being biochemical  inert. As a matter  of facts several pesticides are known to induce   microsomal enzyme systems in the  liver  of fish.  Physiology,  biochemistry and pathology has enabled  us to  assess the  toxicity of the water body with  fish  and in these circumstances, fish can  be used  as an indicator of water quality.