In my experience, most fish are not overly stressed by the proposed change of 2 dGH in the aquarium water. It is still gradual when we change the water in our aquariums. Think of it as the weekly flash flood
. I find it more problematic when people dunk fish in another aquarium with different water parameters without providing time for fish to adapt to the new water ( stress also from moving + new aquarium). However there are other things in the water that influence the osmotic pressure that are not measured by the General Hardness.
The bigger thing here is why would one aim at a specific conductivity/TDS when growing plants ???
Will plants die at 410 µS and blossom at 310 µS ? What is the impact on the plants between changing your water with tap ( say NO3=0, PO4=0, GH=10, KH=8, high conductivity) and (remineralized) RO (NO3=0, PO4=0, GH=3, KH=1, low conductivity) ? I say remineralized as under normal conditions you will run into Mg deficiency pretty fast if using pure RO. Bottom line, it is not the conductivity that matters, it is what makes it high.
I've grown plants using both hard tap water and remineralized RO. Very different values in conductivity, mainly due to different values in GH and KH. In both cases I would notice a small increase during the week due to EI fertilizing, evaporation, by-products. It would revert to standard value with the 50% WC. Difference in most plants was minimal, just needed to adjust my CO2.
Now, some shrimps and fish do require very soft water, and are sensitive to this issue. RO water will therefore be needed from the start . However, the shrimps/fish will respond in different ways if the conductivity increased due to KH/GH increase or due to nitrates build-up or other factors. How sensitive are CRS to an increase in conductivity when KH was kept low I have yet to determine but my guess is that the real problems come from increased KH/GH.