Sorry for the delay, I just got home.
What I am going to suggest, both because of the sensitivity of this species of fish and because of the amount of stress they have already endured, is a drip system for slowly changing your pH in the tank over the course of days rather than hours. This will take a bit of creativity on your part since the water dripping into the tank will obviously need to be above the tank to work, however, this is the safest method I can offer.
When you set up your drip lines, one in and one out, you will need to find a way to anchor them to the side of the tank so the end of the drip line hangs well below the water's surface. I use duct tape for such things as its easy to remove later, but you may have other ideas. Plant weights typically found on bunched plants at the lfs can also be used to weight down the end of each line that needs to be in the water, but there are many options to do the same thing.
In the first bucket, the water that will be going into the tank, mix your RO water with tap water and bring your pH down to .1 below what it is reading in the tank. The easiest way to do this is to start out with a small amount of tap water, test the pH. Add a small amount of RO water and test again. Be sure to thoroughly mix this water before testing and before starting your drip to avoid fluctuations. If you have an air stone you can drop into the bucket to help with mixing, even better. You will also want to test tap water before using it to see if there is any difference between that and the tank water. If there is, please let me know, as this would be an indicator that something inside the tank is raising pH and we will need to figure out what it is so you can remove it asap.
Once you have the bucket water (5 gallons) ready to go into the tank, start your drip going out of the tank into an empty bucket. Using the control valves, set your drip to 1 drip every 1 - 2 seconds, count it out, this is important. Once the drip out has started, wait 2 - 3 minutes and start the drip in. This will ensure that the water is slowly being replaced at the same rate of removal and will never overflow the tank and will provide a very gradual change in pH. As you complete a 5 gallon bucket of change, which should take approximately 1/2 - 1 day, start again by testing the water in the tank, and repeat the process, each time lowering the pH going into the tank by .1... and stop when you have reached a pH of 7.4 in the tank. This entire process will likely take about a week if your tank pH is registering at 7.9. When you get to 7.4 in the tank, maintain that by using the RO/tap mixture for your weekly water changes, and remember to always test the tank water before each water change. Testing is the only way to track and control fluctuations.
(I know this sounds complicated, but once you set up the first buckets and start the first round of drip, it will simplify and become pretty easy) If you have questions, please ask before doing anything so you are clear about the procedure. Changing the pH any faster than that will put all of your fish at risk, especially under the current circumstances. If you need to leave the tank unattended during the course of the day/night, simply turn the control knob to stop the drip, leaving the lines in place for easy start again later. The most important part of this is to take your time and create very slow/gradual change.
I would suggest completing the Prazi treatment before you begin altering the pH, so you have 5 days to organize and tweak your drip process.
In reference to your curing of the rocks... while this seems like a good idea, its important to know that many rocks contain elements and compounds that can/will react negatively to bleach. This can make for a toxic mess for fish, especially in the confines of a closed aquarium system. If you are concerned about the safety of the rock you are using for the tank, it would be much safer to set yourself up yet another bucket with a simple air stone and some tank water (from a water change) and add a few feeder guppies to the bucket. If there is a problem with the rock (toxins, etc) the guppies will let you know within about 48 hrs in most cases. There is really no need for the lengthy process you are using now, which is less safe than the "Guppy
check". If you don't wish to use guppies, ghost shrimp or a mystery snail will offer you the same results, but avoid rams horn snails, as their population can be quite difficult to control and you don't want to send rock with snail eggs into your planted tank.
And, lastly, the floating plant dilemma... again there are many ways to accomplish this. If you choose to use fishing line be sure there is no line running through open areas of the water where the fish may run into it, get hung up on it, etc. Also be careful not to block too much light from the plants below, as this will affect their growth/survival. Tall silk plants that lay over the surface of the water work well at breaking up territory at all levels of the tank, however, they can be expensive. Dollar stores often sell silk plants for much cheaper, but these plants need to be rinsed well, thoroughly dried and then checked for any exposed wires. A simple cigarette lighter can be used to carefully melt the plastic stems over exposed areas of wire... but again, do this carefully so as not to burn yourself or start a fire. I have been using dollar store silk plants in my tanks for many yrs and never had a problem, and they tend to last longer than those made for aquarium use. The wider leaved plants can be cleaned easily using a new toothbrush, and as long as you rinse it with only water when finished, the toothbrush should last for future cleanings almost indefinitely. Do not use bleach to clean silk plants, as this will remove all color from the silk portions and can cause issues if coming into contact with exposed wire.
In the past I have used the wide leaf silk plants around the outer edges of the surface of my tanks, using the long thick stem to hold them in place by simply bending it into a hook shape. This can also be done to hold them between filter intake tubes and the side of the tank glass. If you build rock work up in areas of the tank to reach at least mid level, taller silk plants can also be wedged between them so they reach the surface and still remain in place, which gives you complete control over what areas of the tank become shaded and which ones remain empty enough to allow light to penetrate for the lower live plants.
If you should work with live floating plants they will need to be trimmed often enough to not restrict light to the plants below. I have used this method many times as well, and while it is more beneficial to water quality, it can be a lot more work to keep up with.
Water lettuce is another good floating plant that can be used for this purpose, but again, growth rate must be maintained. The roots of water lettuce will provide a lot of shelter in the upper 1/4 of the tank and at the same time soak a lot of nutrients from the water, which helps maintain low nitrate levels. Water lettuce acts like a nitrate sponge. Please check your lighting to be sure the water lettuce can handle it. Light that is too strong will cause the plants to turn yellow and "burn" them, light that is too low will cause them to whither die off quickly.
When working with the vals and sags, you should know that they like to be crowded to have the best chance at optimal growth. Plant the bulbs in groups of 3 - 5 and do so densely, which will help to encourage new growth that is sent out as a runner beneath the substrate. Also be sure to skim any plant debris from the water's surface so that dead plant matter does not pollute your tank water.
I think thats all for now. If I think of anything else I will come back and post again for you.
Best of luck and let me know if you need any further help.