Nutro Natural Complete Indoor Formula claims to reduce the smell of the litter box. For me Purina One Indoors was the worst. You also might want to change the litter? Maybe your cat is not covering his poop? Anyway: Nutro helped quite a bit; below are the ingredients:
Chicken meal, ground rice, corn gluten meal, rice flour, poultry fat (preserved with mixed tocopherols, a source of natural vitamin E), sunflower oil (preserved with mixed tocopherols, a source of vitamin E), flaxseed meal, tomato pomace, brewers dried yeast, natural flavors, mixed vegetable fiber (carrots, celery, beets, parsley, lettuce, watercress, and spinach), potassium chloride, menhaden fish oil, oat fiber, soy protein concentrate, cranberry powder, choline chloride, taurine, dl-methionine, vitamin E supplement, dried egg product, zinc sulfate, ferrous sulfate, ascorbic acid (source of vitamin C), l-carnitine, inositol, dried bacillus licheniformis fermentation extract, dried bacillus subtilis fermentation extract, lutein, dried chicory root, yucca schidigera extract, niacin, copper sulfate, manganous oxide, vitamin A supplement, garlic flavor, biotin, riboflavin supplement (source of vitamin B2), beta carotene, calcium iodate, calcium pantothenate, pyridoxine hydrochloride (source of vitamin B6), vitamin B12 supplement, thiamine mononitrate (source of vitamin B1), lycopene, vitamin D3 supplement, menadione sodium bisulfite complex (source of vitamin K activity), folic acid, sodium selenite.
Crude protein (min.) 33%, crude fat (min.) 14%, crude fiber (max.) 4%, moisture (max.) 10%, ash (max.) 7.25%, linoleic acid (min.) 4%, calcium (min.) 0.09%, phosphorus (min.) 0.08%, magnesium (max.) 0.085%, iron (min.) 200 mg/kg, manganese (min.) 35 mg/kg, zinc (min.) 250 mg/kg, vitamin D (min.) 1,200 iu/kg, vitamin E (min.) 250 iu/kg, taurine (min.) 0.2%, ascorbic acid* (min.) 50 mg/kg, linolenic acid* (min.) 0.8%, beta carotene* (min.) 3.2 mg/kg, docosahexaenoic acid (DHA)* (min.) 0.06%, l-carnitine* (min.) 150 mg/kg, lutein* (min.) 50 mg/kg.