Your cat’s litter box should not be located near your saltwater tank as the ammonia from the litter box (caused by the cat urinating) will get into your tank and raise ammonia levels.
The rest of the story: While your cat might try to eat your fish by catching them, don’t blame Fluffy for elevated ammonia levels in your tank.
For a cat’s litter box to elevate ammonia levels in your tank, the litter box would have to give off LOTS of ammonia. So much so that you would likely be affected long before your tank. You’d probably have difficulty breathing and your eyes would be burning if you got anywhere close to the ammonia-laden room.
If ammonia levels were this high for an extended period of time, then I might be concerned about rising ammonia levels in your tank. However, if the ammonia levels got anywhere close to a high level, you’d probably notice it and empty the cat’s litter box or at least move it outside.
Also, most cat litter products have an odor (i.e. ammonia) trapping component in them. Therefore any ammonia escaping would be minimal.
My advice: Keep the cat and don’t worry about the litter box being next to your tank. Or why not find the cat a new home…the litter box is taking up space where you could put another (or a bigger) tank!
Special thanks to Brandon R. from my Facebook page for his suggestion for this T.A.Tues.