questions: asked about continous mortality of fish due to water discolouration and haemorrhagic septicaemia 
 answers: first of all perform water quality test: dissolved oxygen, ph, toxic gas(ammonia, nitrite, nitrate, h2s)
for black colour of water, apply toximar @ 5 kg per bigha
for haemorrhagic septicaemia, apply lime @ 10 kg per bigha, use only 50 kg for your 6 bigha pond*
followed by application of kmno4 @ 1.3 kg per bigha for serious condition