Calgary, this is really basic stuff...you cannot argue they get minerals from food and don't need it from RO, it is just basic biology and chemistry as we are not talking about the same minerals.
The reason to take out everything with RO is to get rid of the rubbish and the unknowns you then add back in the good stuff. If you do not understand that then you should not be messing with RO and certainly not arguing about it.
AND can you explain where you get this misconception through that tank water is dirtier than the amazon, my rays are captive bred, they eat gamma irradiated food and I run 2 slow flow uv. Where would these parasites come from? The bacteria that we would worry about could be less than the wild and 99% of wild caught rays would be carrying some level of parasite. Tanks may be pretty nasty in terms of bacteria, many people get food poisoning from tanks but this is not what we are discussing.
RO has nothing to do with bacteria or parasites so don't know why you are bring that up.
I have used RO extensively in reef keeping and understand what it is and what it does, re-mineralisation is the key and fish kept long term in RO straight with no other added water often die of lateral line erosion (where they have one) and other health issues. I can see how some people mix 50/50 and I understand that, it is not for me due to the waste water and I would prefer to use 1 source of water as a known quality, therefore I use hmac.
