Will my mollies be ok if the water stays at its current PH?
No, they will not, and that I will even go so far as to say is guaranteed. However, understand that it is the GH being soft that is also equally significant to the mollies, as this is causing them a great deal of trouble physiologically. These fish simply cannot maintain their normal internal body functions with so little mineral in the water. The pH being acidic is only compounding the problem. But the fish will be in trouble from either the low GH or low pH, if it were possible to raise one but not the other.
Or should I go ahead with the above information to raise the PH slightly to make them more comfortable?
The problem with this is that you are then making life more difficult for the other fish in the same tank. I'll comment more below.
Is it going to be possible for me to find a nice balance in my PH to keep all tank tenants (guppy molly tetra pleco) relatively happy?
No, in my view. This will take some explaining so you better understand my reasoning, and I will conclude with the matter of fish adapting, which is what you are hinting at.
Every freshwater fish species on this planet has evolved over thousands, sometimes hundreds of thousands, of years to function in a very specific environment--which for our purposes here refers to everything from water parameters, to the physical aspects of the watercourse, to other species. Fish are rather unique among vertebrates in this respect, because of their aquatic existence. The internal process that all vertebrate animals use to maintain their health and life (all aspects of "living") is called the physiological homeostasis, or physical equilibrium. Biologists define homeostasis as the tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions. The fish's aquatic environment means that four very important body functions of the homeostasis are closely associated with processes in the gills: gas exchange, hydromineral (osmoregulation) control, acid-base balance [pH], and nitrogenous waste excretion [ammonia]. These processes are possible because of the close proximity of the blood flowing through the gills to the surrounding water, as well as the differences in the chemical composition of these two fluids. This means the fish has a much greater dependency upon its environment than all other animals, and every time that environment changes beyond what the fish has evolved to function in, the fish suffers stress.
I won't go into the ramifications now, but will should you ask. Suffice it to say that stress is the cause of 95% of all fish disease, because stress [as defined by Biology Online] is "the sum of the biological reactions to any adverse stimulus--physical, mental or emotional, internal or external--that tends to disturb the organism's homeostasis; should these compensating reactions be inadequate or inappropriate, they may lead to disorders." So forcing a fish to "live" in an environment that is different from that for which it has evolved means the fish is continually having to compensate, and this takes considerable energy at the very least, but more often results in a weakening of the fish's physiology to such an extent that it finally succumbs either to disease or death. One author compares it to driving a car uphill; it takes far more energy (gas) to maintain the same speed than it does on level ground. Eventually, the fish gives out, because it has no other option.
Briefly on the adaptibility of fish. Some seem to have a greater capacity to get around these problems than others, at least short-term. But one should not assume this means they overcome them, because they just cannot. So the molly that needs minerals like calcium and magnesium from the water in order to keep its homeostasis functioning properly, will not manage in soft water, and similarly the acidity as opposed to a basic (above 7) pH is causing further stress with respect to the acid-base balance. A fish must regulate the pH of its blood to be roughly the same as the water passing over the gills...you can see how this is going to impact things when that blood then starts feeding the fish's cells and organs. Eventually everything just breaks down.
So with that understanding, to your last question...finding a suitable middle ground is next to impossible. While the fish may seem to "adapt" initially, internally things are going on than cannot sustain this on a permanent basis. But having said that, there is some overlap obviously, but I must always ask, at what real cost to the fish? The fact that soft water fish will not live their full life expectancy in hard water is pretty clear proof that they cannot really "adapt" as some would have us believe, and there is at some point going to be a cost.
Byron.
