Let the present volume of liquid present in the fluid be Vi.

3% is pollutant.

After adding fresh water, let the vol. of the tank be Vf and pollutant drops to 0.5%. Since the amount of pollutant remains same before and after adding water, we will take a material balance of the pollutant.

therefore,

(3/100)*Vi = (0.5/100)*Vf

We need to know amount of clean water added, i.e. Vf-Vi

Now, initially we had 2000 gallons of water. after addition of pollutant it becomes 97%.

therefore, 97% of Vi = 2000 gallons.

First solve for vVi, then Vf and finally Vf - Vi to get the answer.