It seems there may be several issues going on many subtle, but taken together, these can add up.
It may be the picture, but the water does not appear to be all that clear.
Here are a few thoughts:
*pH, get this stabilized, do not chase it, using pH often results in bounces in pH
*KH, considering your mix of fish, I would make sure it is at least 100 ppm or higher via a good buffer
*Make sure your GH is at least 200 ppm with mollies
When changing water make sure the pH is similar, and make sure the water is mineralized, do not use RO, DI, Drinking Water, etc.
Good bio filtration is a must, sponge filtration is a simple but effective bio filter, with FSB filters a step up
As mentioned to you earlier, changing out filter cartridges can start a mini cycle, adding a sponge pre-filter to your filter intake can help with this, though this will not be overnight
As noted, separating fish would be a something I would do right away
Medicated Baths using 1-2 teaspoons of salt per gallon along with Methylene Blue (1/4 teaspoon per gal of a 2.303% solution) for 30 minutes 1-2 times per day may be helpful, while not dumping meds into your main tank
SeaChem Stability or similar products may be helpful if your ammonia is not staying below .5 ppm.
You should Not have to make water changes for the reasons of keeping ammonia levels down in a healthy aquarium with a well established bio filter