This is not likely to be a low-oxygen issue. Though having said that, other issues can cause less oxygen, but then those other issues need to be addressed. I concur with Colin's advice above, but there are other problems here to be considered going forward, and these might well be part of the problem now.
First is the GH, at 8 dGH (I assume this is degrees, not ppm or mg/l) the water is too soft for mollies. They require the high calcium and magnesium to be healthy long-term and this means a GH of minimum 10 dGH but preferably higher. This has a significant effect on mollies--all livebearers actually, but mollies are especially sensitive to this. It is a bit soon for this to be causing serious issues but it is weakening the fish and this is a contributing factor and may for all we know be thee issue.
Second thing is that a tank of 66 liters (17 gallons) is very small space for mollies. This is not the issue here, again, but in time it will become another.
Last popint is the balloon variety--this poor fish is deliberately deformed to create the balloon shape and it does seriously damage the fish internally. These fish often have shorter lives; some say they can feel pain, some disagree, but all agree this is not an acceptable practice and the only way to stop it is by not buying balloon fish of any species where this is being done. It also obviously weakens the fish's physiology and this will mean other problems, so this too is a factor.
