cleaning up a dirty bowl

Question
My son brought a tank with a black moor that's ten yrs old.
the water is horrible, brackish if that means brown! He says he just adds water to the tank, no filter. And that's what he did.  I'm really not happy with this dirty water environment and would like to clean up this poor fish's tank.
Can I buy spring water, clean the tank and replace the water?  Will it shock the fish to have clean water if this has been his "normal" for all these years?  Believe it or not he's active and yet I can't bear looking at this dirty water.  Thank you in advance.  marjie

Answer
Brackish means a salt/freshwater mix. Yes you can buy spring water and clean the tank however make sure the water is defiantly pure and that you use no chemicals of any sort when cleaning the bowl. Just put him in a plastic bag with the water hes been kept in and have the bag float in the bowl, this will allow him to accumulate and not be shocked, hope everything works out with you, Alex