Both cats suddenly hate food?

I have owned a cat for nine years, and have fed him Iam's normal adult cat food for 8 years. This Winter I adopted another adult male cat who, according to my aunt, has also been fed Iam's normal adult cat food. Suddenly both cats will NOT eat anymore food than is necessary not to starve to death. I have to throw away bowls full of food every week because they simply eat very little and then the rest is banished to stale status. I even got to the point I was feeding them 1/4 a cup of food 4 times per day, and the result was exactly the same. So I did the logical thing, and switched them to Blue Buffalo food, with the same result, and Natural balance; same result. I am at my wit's end here. My older cat has never liked canned cat food all that much; he will eat it but leaves a bunch to dry out. What can I do?!

Maybe a trip to the vet is in order?

I find my cats lose interest in their food if I feed them the same thing all the time. I don't believe the dry foods you listed are the best. Good luck.

Vet visit. Lack of appetite especially in an older cat can be a sign of a medical issue. I also have an older cat that hates wet food, but I transitioned him to mostly raw food successfully. It's very important that you get your olde cat on some kind of wet food, whether its raw, freeze dried, or wet food. Many picky cats love freeze dried meals like Stella and Chewy or Primal. Kibble should not be fed exclusively or you risk kidney issues. 

I agree, a Vet visit is in order..

Anytime a cat refuses to eat multiple different variations of food, then it is Vet time. 

When it is an older cat~~ it is a necessity ASAP!

When there are 2 cats not eating the same foods ~~~ Then it is an Emergency, Urgent appointment...

Their appetite is fine, I think they are just spoiled. They did refuse to drink water out of a bowl when I introduced them to slow drip bathroom sink water. They wine and passive aggressively sit next to their food bowl and when I put some in they kind of act disappointed and nibble for like 30 seconds. They even went as far as to open the sink cabinet door and knock the garbage over to try to get human food out, both food bowls half full ofc.

I tried some blue buffalo wet food yesterday and they were excited at first and then left it for a while before finishing it. Then later last night I opened a can of tuna and surprisingly they both ate it right away. I hope this progress. Ofc the younger cat was crying about the bowl being empty an hour later. I guess only close to a half pound of food in a day is torture -_-