what I believe @Colin_T is saying is that he doesn't think there is a "happy medium" of hardness that the fish species can coexist in, thus it would be better for their long term health to be kept in an environment that most accurately matches their species' natural hardness.
With regard to the original question, when moving the fish the best approach would be to try and take as much water with you as you are able to (for example, canisters of fresh tap water from your tap) then slowly acclimate the fish (over the course of a few weeks) to a new mix of RO/DI water + tap water from your new home to match the original hardness, or whatever hardness you are targeting.
With regard to the original question, when moving the fish the best approach would be to try and take as much water with you as you are able to (for example, canisters of fresh tap water from your tap) then slowly acclimate the fish (over the course of a few weeks) to a new mix of RO/DI water + tap water from your new home to match the original hardness, or whatever hardness you are targeting.