Darting flashing new molly addition to a mature tank

I have a 55 gallon planted low tech tank up for several years with only a few old fish( bristle-nose pleco, and one oto cat or yea and one very old amano shrimp. The tank has been ignored and its amazing how well things go with little intervention( often better than when I intervene).I did my monthly water changed and arranged some of the Malaysian drift wood into arch patterns... this rejuvenated my interest and so I purchased a few weeks latter 2 lyretail mollies - one gold and the other marble, The gold one's body seemed off but the lady at the LFS was having trouble so I kept it. It has shimmies ( and probably had it at the time of purchase). It now is flashing and occasionally darting but otherwise is very active eating and dorsal fin is rarely clamped down. My tank parameters are stable - checked prior to getting new fish. My water is suitable for live bearers naturally ( GH ~ 7-8). The other fish remain symptom free. Although the tank I got the fish from was certainly not the healthiest and the fact that I did not QT my new fish leads me to believe #1 Stupid me...why did I do this impulsive thing and #2 I believe these symptoms are a precursor to Ich since it was present in some of the tanks at the LFS ( P......T). I gave my old tanks away long ago to some neighbor kids so I have no QT (I have not bought a fish in years). Would you guys wait to see what evolves or treat with salt/heat? I believe removing the fish after 4-5 days is too late from a tank/infectious standpoint. No white spots and my water temp is 77F. No gasping, fin defects, or body sores. Gills do not look inflamed . I understand the life cycle of ich... so should I wait to see the proverbial spots before my eyes before I worry/treat or should I be proactive???

That is what I am doing- thank you. I did get a ten gallon and will set it up a QT just to be ready, Correct me but if I do use heat and salt in the QT tank, I do not need to remove the carbon filter( I used my established tanks media/ water to quick cycle this tank) .