If the nitrates are high enough to be a concern then I would up it to at least 50% once or even twice daily until they drop. More if they are extremely high. IMO 25% won't be making much of a difference. If your tap water contains nitrates then I would look into some fast growing stem and floating plants to help with the problem. The pleco in the picture has fin rot from what I can tell so the lower the nitrates and the cleaner you can get the water the better.
And how do I cure the fin rot!?
For fin rot you just need some nice clean water. The 50% pwcs that you are doing for the high nitrates should suffice to keep your water clean and help you fish' fins.