Hmm. Several things aren't quite jiving here, and there are a couple of red flags. I know you didn't ask about anything but the fish oil, so I'll answer that first, but I would feel remiss in not addressing my other concerns, as well.

Fish oil is a great supplement for cats and dogs and can help improve coat quality, as well as support the cardiovascular system. I puncture a softgel and squeeze 3-4 drops of fish oil on each of my cat's breakfasts every other day.

The bigger issue, though, most likely relates to your cat's diet and/or state of health. C/D is not a kidney diet. It is formulated to prevent the formation of urinary struvite crystals. K/D is the kidney diet. Both diets are loaded with useless carbs that are not doing your cat a bit of nutritional good. If you want to improve coat and overall health, consider switching to a high quality canned food, or at the least to a low-carb dry food like Innova Evo or Wellness Core.

Dandruff is common in obese or older, arthritic cats who cannot groom properly. It is also common during the dry winter months (a humidifier can help in this case). It may also be symptomatic of disease.

What concerns me about your description is that your cat is somewhat skinny, has a dry, dull coat (which may indicate dehydration), is drinking a lot of water (which may also indicate dehydration), and has a history of renal problems. When was the last time you had a full blood chemistry run on him? Have you had his thyroid checked? If you haven't had these things done within the last 3 mos, I strongly recommend you do them now. Fish oil isn't going to fix renal failure or hyperthyroidism. If your cat is dealing with one or both of those disease processes (or any other disease), the sooner you know about it, the more likely you'll be able to manage his condition satisfactorily.

I wish you and your boy all the best.

Laurie