It's the nature of the beast with silver dollars and pacu's, the waters they are endemic too are very unforgiving with predators; so their natural response is to flee in the wild, and in tanks. Unfortunately this can have a detrimental effect on other fish, indicating to them that there is an apparent danger nearby, can be very unsettling to fish that prefer slow moving tankmates.
Pacu's get absolutely massive, so it would be prudent to evaluate how you'll deal with that in future; if the answer is that you can't, I'd recommend doing something about it at this stage, as it will be infinitely harder to deal with it when you have a huge, huge fish on your hands. This also applies to the RTC, clown knifes are also heavyweights.
Not trying to discourage at all, just trying to help out with info