Am I getting this correct, you change 50% of your water? If so, how often would that much of a WC be required for good water management practices. I actually changed around 12 gallons, in a 125 gallon, and thought I was in the ball park with a 10% change after two week, and then another 10% in two more weeks. So 20% in a month. If I changed 50% that would be 60 gallons at one time, after a month, I need to remove all the chlorine from the water. What is the best, and cheapest way to treat 60 gallons of tap water for chlorine removal at a time? Even if I used 30 gallons from the RO, and 30 from the tap. Treating 30 gallons a month for chlorine removal has a significant cost? I have not looked for test kits for chlorine, do they make them? Is there a control limit for amounts of chlorine. I figure zero would be a good number for the fish. We all squirt our plants with house tap water, out of the hose bibs, which has chlorine. Are the aquatic plants more fussy with chlorine levels?
I have done 50% water changes on all my tanks for 15 years. I have a lot of fish. The lighter the fish load, along with the more plants, the less volume is required. Some sources advocate near-zero water changes. But the tank has to be balanced biologically, with a moderate (light) fish load, lots of plants, and controlled feeding. The more fish, the less this will work.
All sources (at least those who recommend regular water changes) do agree that water changes should be every week, no less. It is easier to maintain stability with a weekly partial water change, again assuming a balanced biology. The volume can vary, depending upon the fish load (species, number). I would suggest 1/3 to 1/2 the tank weekly.
Short comment on discus and water changes. Many discus aquarists do two or three water changes a week, changing 1/3 to 1/2 the tank at each. Discus breeders do 95% water changes every day, sometimes 2 or 3 times a day. Water changes are not detrimental, provided parameters are the same between tank and tap. And in your case, that is why I recommended checking the pH. One can always do more changes and change less volume when this is an issue. But I wouldn't see that in your situation, any more than mine.
Why do water changes? The answer is, it is the only way to remove substances that naturally accumulate and that cannot be removed by any other means, including filters. Urine from the fish, liquefied waste as processed by bacteria in filters, pheromones released by fish and chemicals released by some plants [allelopathy]--all of this "crud" as I call it can only be removed by removing the water. Plants can handle all this, but their capacity is not sufficient to handle a large fish load.
Treating water. Tap water should always be treated with a good conditioner. Chlorine is present in all municipal water supplies in North America (as far as I know), and many municipalities also add chloramine which uses ammonia to treat the water (kill bacteria) longer than chlorine does. Chlorine is a gas and as the water flows through the pipes, it gasses out of the water, so eventually it will be gone. Here in Vancouver they solve this problem by adding chlorine at various substations along the way, to keep it at higher strength. You can remove chlorine by briskly agitating the water, or letting it sit out overnight. But this does not remove chloramine, which can only be detoxified chemically. Most aquarium water conditioners handle chlorine and chloramine. Many will also detoxify heavy metals. Beyond this, there are some that detoxify ammonia, some nitrite, and one or two handle nitrate. All you need is one that deals with what is in your water. I personally see no point in using chemicals to deal with substances that are not in the tap water, or that are best handled by the plants--which is why we have them.
Water conditioners are really not very expensive. I by the largest jug I can, usually 2 liters or 1-2 gallons, depending upon manufacturer; I pour some into one of the standard-sized plastic bottles with the squirt top so it is easy to use. I only need chlorine removal, so any conditioner would theoretically work for me. I get the cheapest that does the least possible. Buying these online can save a lot of money too. And you can stock up, they do not lose their effectiveness. I had a bottle opened in the cupboard for 10 years and it still did the job--and I have a lot of chlorine in my tap water.
I use a Python hose hooked up to the laundry sink (needs a screw-type faucet) to drain the water out and then refill the tank straight from the tap, using cold and hot at approximately the same temp, usually slightly cooler, than the tank. I squirt in the conditioner as it starts to fill. I do this for my 115g, 90g, 70g, 33g, 29g and 20g tanks. The 10g I use a bucket to fill as the hose is too fast in so small a tank.