My roommate has a cat, just one. We have two kitty litter boxes in the house. We live in an area where outdoor cats run a very high risk of being eaten by coyotes, so Kooks is in a indoor animal. As we were sitting around yesterday having a conversation my roommate said, "That smells, I'm going to clean out that litter box!" Which brings me to the number one way to reduce odors pet odor in your house: keep the boxes clean! Having clean litter boxes available for your cats encourages them to use them. No matter how many cats you have, they will eventually get disgusted by a dirty litter box and start looking other places to poop. This can be quite nauseating when you discover stuff under your couch, behind your bookcase, under the bed etc.
Making it a daily chore will also make it less gross. The more often you do it, the less there is to do. Get a scooper from any pet store. Another way to cut down on odors is to limit the number of animals you have. My friend has eight cats and one small dog. She has plenty of litter boxes strewn around her house, but the odor is still eye watering. I am sure if she got herself down to three cats, with or without the dog, the house wouldn't smell as much.
There are other options. Recently a friend of mine bought a device that trains a cat how to poop in a toilet. If you do that, then all you have to do is flush it away when you see it! There are also devices that take the stuff away through your septic system, Cat Genie, is the name of one. Another option is to have outdoor cats. When I lived in Waialua in Hawaii there were no coyotes. All my cats were outdoor cats. There were still plenty warm and friendly, and showed up like clockwork to get their food. I was certainly fond of the outdoor cat idea, they keep down mice.
Another way to keep down the odors is adding baking soda, an odor absorber to your cat's litter box, and ruffling it into your dog's fur. Frankly dog's smell less if you give them baths, except they don't like baths. Dogs have a very different definition of what smells "good" from us. Often the stronger, and the more foul the smell, the more dogs seem into it. Some dogs are also, more naturally smelly than others. My friend was quite enamored of the idea of owning a Shar Pei. With their cute wrinkled skin, she thought it would be an adorable pet. After her boyfriend surprised her with one she was quite horrified by the strong smell! She ended up giving it away.
A word on what doesn't work: Air fresheners. Spraying something like Febreeze around works for exactly as long as the can in your hand. So that's not a good permanent solution. Never mind that the ad shows a happy mother returning again and again to her son's closet to smell the Febreeze. In real life it doesn't last. I used to spray it on upholstery when I worked as a housekeeper at Camp AREV. Nothing I did kept those couch pillows from smelling musty. There were just too many bodies sweating into the fabric. The plug into an outlet air fresheners are not much better. They last longer, however, they only mask pet odors slightly. They don't erase pet odors the way opening a window will.
If you live in a place where it is safe to keep the windows open, fresh air will do wonders as far as eliminating pet odors. Unless of course, you have a neighbor like my prior neighbor, "Freddie." He owned about five dogs he kept in a small yard. When the wind blew the wrong direction it was rank! Eliminate pet odors by getting rid of the stuff! If you pick it up and take it to the dump it won't smell so bad. It's pretty common in cities and dog parks to see stands with plastic baggies for the purpose. You can do this on your own property as well. As with the litter boxes, the more often you do it, the less intimidating the task is. Just get on a schedule and think of it as no different than changing your kid's diaper. Dog's like a clean spot as well.
I have bought charcoal for the house which was supposed to have odor absorbing qualities. I did not notice that the charcoal had any effect. I have also purchased an air purifier. This product actually did a great job. Not only did it erase all traces of animal smell, it also got rid of the smell of cigarettes! No easy feat! It makes kind of a white noise while it's running, and did not raise our electric bill substantially. Of all the tricks to reduce animal odors, the air purifier was the most successful.
Fish are an animal you may not think has a lot of odor, but they can if you don't keep the tank clean. I recommend buying at least one of those fish called "algae eaters" to keep the algae down and changing the water on a schedule. Once a month or more often if you live in a warm climate and the tank starts to stink. Reptiles and frogs can also get smelly if you don't clean their aquariums. My former husband kept a Chinese Water Dragon as a pet, and he completely changed the tank every couple of months. It was fun as well as interesting and definitely kept the odors at bay.
Birds are probably the least smelly animal to keep around. They need clean newspaper for the floor of their cage. You can sprinkle it with a little baking soda to absorb smells without harming the bird. You can also put a piddle pad (designed for dogs) on the bottom.