Thank you for this information. Now I can explain exactly what's happening, since what you provided makes sense. This may be a bit long, but it is important to understand what occurs in an aquarium, and as you have seen, things can occur quickly to the detriment of the fish.
The pH is the measurement of the ratio between two ions, the positively-charged hydrogen ion and the negatively-charged hydroxyl ion. Acidic water has more hydrogen ions, alkaline water more hydroxol ions. Neutral water is pH 7, acidic is lower and alkaline higher on this scale.
Fish, plants and bacteria release CO2 (carbon dioxide) continually; more comes from various bacteria than the fish and plants. CO2 produces carbonic acid, and this causes the pH [which is the measure of how acidic or alkaline water is] to lower. As organics [fish waste, uneaten food, animal and plant debris, bacteria dying, etc] increase, more CO2 is released and thus more carbonic acid. It is non-stop.
The pH is loosely connected to the water hardness. Hardness is the measurement of mineral salts in the water, primarily calcium and magnesium, but others also impact hardness. Carbonate hardness (expressed as KH] is the amount of bicarbonates in the water and these act as a buffer to pH. It does this by reducing fluctuations in hydrogen ions, thus preventing the pH from lowering. But at some point the buffering capacity may be reached, and then the pH can suddenly fall, what we term a pH crash.
According to the info you have provided, your water has zero KH, which means no buffering at all. The aquarium will therefore naturally become more and more acidic, and the pH will lower. This will be steady, but not sudden, and depending upon the fish species they usually adjust. In your case, having mainly soft water fish, they are generally OK with acidic water. But it can get too acidic.
We prevent this a couple of ways. One is regular water changes. The more water changed and the more often, the more stable the pH will remain. Provided something else doesn't impact it, such as overcrowding which increases the organics beyond what this can handle.
Plants also work to keep the water stable. They do this through their diurnal cycle. In a relatively-heavily planted tank, the pH will be higher at the end of the daylight period and lowest at the end of the darkness period. I wrote at length about this in another thread, and won't repeat all that here, but you can read it if you're interested in the process; it is post #22 in this thread:
http://www.tropicalfishkeeping.com/freshwater-aquarium/ph-levels-pretty-high-68972/
The pH adjusting chemical you were using clearly was inadequate to keep up with the increasing CO2. Normally these adjusters work fairly well when the KH is non-existent, but even if it did, I would not continue to use chemicals. The fluctuating pH is highly stressful, and other side effects from the chemicals can be even worse. My thinking is always to put as little stuff in an aquarium as possible, and let nature do the work which will always be safer and less expensive.
On the plants, those are actually terrestrial plants, not aquatic. That explains their minimal impact on all this. True aquatic plants would use most of the CO2 and help prevent lowering pH. And, those plants will start to rot and cause even more acid. I would remove them and replace them with true aquarium plants. Many of the commonly-available species are included in our fish and plant profile section [second tab from the left in the blue bar across the top of the page]. Have a look, and ask any questions and one of us will be able to answer about the plants. At this stage, I would recommend fast-growing plants like stem plants because they use a lot of nutrients including CO2 from the water.
The bubbles from the rock was merely air escaping; rock is sometimes very porous. Not an issue there.
Please stop using the Waste Control. This is only making things worse, and it speeds up the decomposition of the organics adding yet more CO2. It also releases ammonia, although this is somewhat harmless since in acidic water ammonia changes to ammonium. I won't go further into that.
The water conditioner is sufficient, with each water change, enough for the replacement water volume. Plant food is fine, but as I say you need some true aquatic plants. StressCoat I would also stop, this is just another chemical mix going into the soup, and it is not needed.
I would increase your water changes to 30% and one every 3 days for a week or so to slowly stabilize the tank. This will be slow enough to not harm the fish.
Byron.