Yes there are lots of exagerations, but mostly there are illogical or irrational conclusions drawn from circumstantial evidence. Most people don't even know what pH is. I mean, have you actually thought about what pH is actually a measure of? It's just a ratio of one ion to another. In fact pH can be an acronym for "percent of Hydrogen ion (H+)". Low pH means high concentration of H+, and this can only happen if acids are added to the water.
In a tank acids are produced just from the breakdown of waste products. If you clean you tank regularly then this waste is removed, but even if you don't the acidic level of the water can't just all of a sudden fall off a cliff unless you dump acid in the tank yourself.
Most of the fish we keep are from highlky acidic waters like The Amazon River Basin where the pH of some streams can fall to 2 or 3. So even if there was suddenly a crash these fish would not be affected in the way it's claimed.
Here is an example of a tank where the KH was kept at 2 and the pH kept at about 3 with a combination of RO water and CO2 injection (The Apistogramma in the middle was breeding in the tank.):
There was no crashing or melting or any of that. Flora and fauna just carried on. This does not mean that plants never melt or that fish never suffocate, but when they do, it's not because of pH crashing. Plants melting occurs when they are starved of CO2. Fish suffocation happens when there is too much CO2. But we have not had a very accurate way of knowing the CO2 levels in our tanks - not unless we spend thousands of pounds buying scientific equipment to measure it, so people don't really know what the CO2 levels are and since they cannot measure it they attribute all the faults they observe to something that they
can measure. This is like losing you purse while walking down a dark alley and then looking for the purse on the High Street because there's more light on that street than in the alley. Doing that, you'll never find your purse and you'll never solve the problems in your tank by blaming everything on pH.
In any case, there's nothing wrong with buffering your water some more with bicarbonate. It's not really a big deal and it does no harm. It's the misunderstanding that does the harm.
There are really not that many plants which really need to be in soft water - only a handful, really and the some of these do OK in hard water (I haven't tried all of them). On the other hand, soft water is not what does damage to the so called hard water plants. The reason that some plants don't do well in soft water has to do more with the nutrient levels which are typically kept low by those who want to maintain the low TDS. So again, cause and effect gets confused because the softness gets blamed when the real culprit is nutrient deficiency, which you have noted with your Phosphate situation. Vallis is grown at many different KH values, however, in non-CO2 tanks, when the CO2 drops below some threshold value this plant is able to use bicarbonate to produce CO2. In a CO2 injected tank Vallis does not care about the KH because there is plenty of CO2.
The most important items in a planted tank are CO2, nutrients and avoidance of dibilitating excess of lighting. These have the biggest impact and pH/KH/GH has relatively minor impact.
Cheers,
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