Mono agression

[Talleyrand]

After a brackish tank fell apart several months ago I added the last survivor into a 55 US gallon freshwater tank. For the last few weeks the Mono has taken to occasionally chasing the three silver dollars that share his new home. None of the dollars seem worse for wear, but it's starting to bother me. I am considering adding one or two more Mono's hoping to relieve some of his agression, though I dont really want to add more brackish fish into a freshwater enviroment.
The only reason the Mono is in the tank is I have no where else to house him and the local lfs wont take him.
My wife refuses to let me have another tank of the size this fish would require.
So........
Does he go?
Get buddies?
Something else?

 

[CFC]

Without salt the mono wont last long, although they do enter freshwaters this is only to spawn and it is at most 2 or 3 months before they head back out into the ocean where they feed in costal waters. Monos are schooling fish so it is natural for them to seek out companionship with similar looking fish, i would guess that the mono is simply trying to school with the silverdollars but they are not keen on the idea so are constantly swimming away.
If you really cant get another tank for the mono to live in a brackish enviroment then i would advise finding somewhere/someone that can cater for brackish fishes needs.