LOL its physically impossible for your RO unit to make water harder (unless it's made from limestone...or in fact not an RO unit and actually a Kalk-wasser unit)
Also have you tested your tap water post RO once it's been mixed with salt mix to the correct salinity? I cant tell that from what you've written.
Carbonate Hardness KH (Temporary Hardness)
As well as calcium and magnesium, most natural waters also contain bicarbonates as part of the total salt mix. It is these bicarbonates that make up the carbonate hardness or KH value. The bicarbonates have an important function in the overall chemistry of aquarium water. They act as a pH buffer preventing sudden changes in the pH value. Where bicarbonates are deficient ( i.e. low KH value), large fluctuations in the pH can occur under certain circumstances and the addition of a small amount of acid or alkaline water will cause a drastic change in the pH of the aquarium water. The higher the KH the better the pH buffering, but a very high KH can lead to a markedly alkaline pH. Carbonate hardness is also known as temporary hardness because bicarbonates change to carbonates and come out of solution when the water is boiled. Recommended values of the KH for stable conditions are around 2° to 8° dH, although certain fish do have other requirements.
What to do if the KH is too low
Make up a strong solution of sodium bicarbonate and add it to the water a little at a time, until the desired KH is reached. Ensure water is well aerated during this process. Do not add the sodium bicarbonate solution directly into a tank containing fish or plants, as the sudden change may adversely affect the fish and plants. The adjustment should be made to water outside of the tank, and the adjusted water then added slowly to the aquarium. This should be carried out slowly over a period of several days, closely monitoring the pH during the adjustment.
What to do if the KH is too high
Dilute the hardwater with clean rain water, distilled water, previously boiled water, or water treated with a commercially available softener (only use this if you intend not adding livestock for at least a month). If practical, the tank can be filled with boiled water that has been cooled and aerated
