If you must give it in drinkng water and you want to ensure that each animals atleast drinks a sizeable quantity of the DENA containing water then you may have to put each animal in a seperate housing. Alternatively you can administer intraperitoneally at either 20 mg/kg thrice a week for 14 weeks or at 200 mg/kg single dose; below is link to two articles that may help
Serum N-glycome biomarker for monitoring development of DENA ...
www.ncbi.nlm.nih.gov/pubmed/20704698
by M Fang - 2010
Curcumin / BSA: New Approach for Hepatocellular Carcinoma Treatment
You may think of combining DENA with phenobarbitol for faster induction These articles may helpp; Chemically Induced Hepatocellular Carcinoma and Stages - InTech cdn.intechopen.com/.../InTech-Chemically_induced_hepatocellular_carc...
What is more important is not for the rats to drink 'same' volumes, but for the right doses of the induction agent to be delivered. The doses have been given in earlier posts, but remember, it is the doses that you are concerned about, not giving uniform volumes.The only way of being 100 % sure of uniform volumes is an individual fixed-volume calculation of dose, which will be delivered by gavage.