Hi and welcome to the forum
GH is the general or total hardness of water. It is basically the level of dissolved calcium and magnesium in the water. Salt does not affect GH.
While salt is often suggested for a remedy with mollies, it is not going to help when the GH is soft. You haven't given the GH number, but the acidic pH suggests the water is not on the hard side, and this is the issue with mollies (indeed all livebearers, mollies especially sensitive to soft water).
Do not mess with the pH. First, this is tied to the GH and KH and unless those are adjusted the pH will not budge permanently. And fluctuating pH is very stressful and damaging to fish.
It is the GH we must know here, and then look at options. We also need to pin down what the "catfish" is, as it is likely suited to softer water and increasing hardness may harm it.
EDIT. Your edit in post #4 appeared after I posted. As mentioned above, the GH and KH are connected. Using pH adjusting chemicals will temporarily increase the pH but within 24 hours (or less) the GH/KH return the pH to where it was, depending upon their level. And these adjusting chemicals are not easy on fish aside from their effectiveness or lack thereof.
You can purchase mineral salts (these are salts of minerals like calcium and magnesium, not "common" salt sodium chloride which is what sea salt and aquarium salt is), or you can use a calcareous substrate (like aragonite sand). But this will not be appreciated by any soft water fish, and we still do not know the catfish species. And the tank size...mollies need spacious tanks.
Any chance of a picture and short 20 second video of the fish and tank?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
If the video is too big for this website, post it on YouTube and copy & paste the link here. We can view it at YouTube. If you are using a mobile phone to take the video, have the phone horizontal so the video takes up the entire screen. If you have the phone vertical, you get video in the middle and black on either side.
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What are the tank dimensions (length x width x height)?
The GH (general hardness), KH (carbonate hardness), and pH of your water supply can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
What sort of filter is on the tank?
How often do you clean the filter and how do you clean it?
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Do a 75% water change and gravel clean the substrate every day for the next 2 weeks or until we work out what is going on. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media in a bucket of tank water and re-use it.