If you had a water softener your KH would be very high and your GH would ve very low or zero. Your current water parameters are consistent with rain water.You state you are a condo owner. HOA don't put in one water softener for the entire complex. It would be very expensive.
Rapid changes in water however likely your problem. While I have been aware that water change like this can happen. I never expected it would occur as rapidly as it did according to vvDO indicated. Most advise regarding aquarium depends on your tap water being resonably stable and consistent. If it isn't it can be very had to get consistantly good plant growth. About the only way to deal with this is to measure GH and KH before each water change and then add a GH booster as necessary to maintain a stable GH. Doable but you have to test once a week or which takes time and then calculate the dose needed to get to your target GH and then mix everything and do the water change.
Most under sink RO systems don't take up that much space and they can produce about 25 gallons a day. Most don't have a storage tank for that much water but if you have enough space you can get a pressurize tank separately. Or you could put enough in a bucket after a day or two for the next water change. This is what I do for my small tank.
My location is accurate so my water is likely the same as the previous poster as we don't have a lot of water district in my area. In a state where we have massive drought, then massive rain, I don't really know how that changes things, especially since unlike a smaller city, we have many water reservoirs and I don't know how it all works. Just going online real quick, we have many in the area, but also have some 90 miles away in the mountains. That's why I haven't relied heavily on test kits as I have never felt like my water is consistent as the early books I read, or what I now read online. I know that in the past, I have had changes in my parameters from the same water source, when I tracked things more concisely, but rarely did that help me make better decisions. That said, did my water change? Well, I did one test wrong, but others right and they changed, and in the same time period, I just don't know how rapidly. I do know I had some really vibrant reds only 2 months ago and now they are far from that color.
I know nothing about water softeners but my first guess is, my HOA wouldn't add them, and if they existed previously, I would imagine they would take them out, our HOA got higher than we wanted so any unnecessary cost and maintenance, we would cut out. I can still ask, but it's hard to get that info easily.
As for an RO unit here, its a little tough. I have about 650 ft, not including a long hall way, in a one bedroom, with no outdoor plumbing and its not really an option to me, unless I want to fight my HOA, and pay companies I don't trust, who do what seems to be work that requires constant repairs, at a higher cost than other plumbers (I inquired for much more minor plumbing work and it was stupid expensive).
If I do RO, it will be from my LFS. Even a 5 gallon container is hard, my closet shelves are too short and up higher (which is a concern in CA with Earthquakes) and I don't want them out in the open. If it is a must, I would do it.
At the end of the day, its not that my plants are not growing or my tank is really suffering, not at all. My coloration is just not what I want. I trim 1/2 of a gallon pitcher regularly.