You can do small changes over time so it doesn’t stress out the fish. Eventually, the aquarium will be closer to 50ppm.
If fish, most fish that is, are not acclimated properly to the concentration of things other than hydrogen and oxygen ions in water (the collective: everything that is dissolved in it) and in the contrast of water before and after is sufficiently large, then there exists a chance that osmosis will get to them and will eventually kill them. It is a relatively slow process, but death is the outcome. Now, having said that, there exist fish, which can survive extreme swings and actually need close to that to breed. Corydoras and plecos for example. But we are assuming that we are talking about sensitive fish, like (extreme) Discus. What I cannot give without looking things up is solid numbers of actual lethal TDS difference(es) would be and for what exact fish. Seriouslyfish.com typically address those things for a given soecies, if it is something to watch for. The experience is not pleasant, as you imagine and it is a very good question to ask. i have killed fish in the past; Chocolate Guramis to be exact. Pays to do extensive research ahead of buying fish. Only to find later that they are difficult to keep, which was NOT what the store (which I no longer go to) told me. Ever since then I have mastered the art of drip acclimation, which might be a way for You to imply. It just takes a very long time, if we are talking large volumes of water.I'm wondering how close I should match my tank TDS to the water going in?
For example; tank is 150 ppm and the water going in is 50 ppm...would I try and get the water going in closer to the tanks 150 ppm or don't adjust it all?
I know it depends on fish and plants for what's right for me but wasn't sure about there being what's to me a big difference...if I'm wrong please let me know😂