What filtration have you got installed, how long has the tank been running and how often do you feed your fish?
Your immediate problem is caused by an insufficient amount of bacteria to deal with the amount of waste that is being produced. Both plecs and predatory meat eating fish like rtc's produce a lot of waste. If your filtration is not adaquate enough to cope with the fish waste and any rotting left overs then that will explain the ammonia spike. To rectify it, you'll either need to increase the biological capacity by adding another filter/or upgrading your existing one or reduce the amount of food you are giving your fish. Increasing water changes will help in the short term and provide some relief for the fish but it won't solve the problem.
What else are you feeding him other than salmon and where does this salmon come from, ie lfs, tinned from a supermarket? Another rtc owner would be able to advise you better but I would of thought salmon, while good as a treat is not a terribly good main diet. Although, the existence of ammonia in the tank may well explain his loss of appetite.
How big is the plec and do you know what type of plec it is? With the minimal amount of information on the tank that you've provided I would guess the problem is territorial. A 14" rtc I'd imagine uses pretty much all of a 6ft tank and the plec I'd guess just wants to be left alone. So he's constantly having to chase him away from what he classes as his area (which is somewhat difficult when you're 14" in size and only have 6ft to roam). Providing a bogwood construction where the plec can get away from the rtc might solve your problem (assuming it is territorial) but owning an rtc means you'll eventually need to get a far bigger tank than the one you currently have if you wish to house him properly in the future. Although many would argue that that time has already come.
