Should I use RO/DI?

Yeah, I agree with TTA...to a point. Many of our fish are farmed nowadays, so be careful what water parameters are important to the fish you will acquire. You can ask (or test if local) for their tank parameters. Many of our fish are happy at a wide range...but not all. Did you say what fish you're hoping to keep? Do you hope to breed?
 
If you wanted what I would do is like 50/50 rodi depending on your water and do Water changes once in a while With same water and when your water evaporates (you want it to happen for this) you can do a rain drip of colder rodi to simulate rain. usually in most scenarios ph and tds don’t matter except with like wild bettas or wild apistogramma that only ever get imported like once every decade but my rule of thumb is I keep my tanks on the dirty side so I get meiofauna and microfauna which are food for my fish and will keep the top of my substrate from getting anerobic and releasping hydrogen sulfide stuff like crushed bio balls in your substrate can do this too.
 
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