Hi all, newbe here.

[Byron]

Hi Guys, due to my high KH/GH levels (source water issue) I have been looking into RO units.
Is it possible to buy a small portable unit & connect it to a submersible pump to create a loop that takes water from my tank & puts it back in with lower hardness.
Think most units need 3bar to run but I'm unsure if small pumps can generate this.

I don't think there is anything like this, but even if there is, you would not want to be doing this as it is very hard on the fish. The water they live in must be stable, and thus the parameters must be adjusted outside the tank before you add it as "fresh" water.