| Ivar,
Remember that TDS represents the total amount of all things disolved in the water. On the other hand things suspended in the water (not desolved) do not affect the number. Distilled water would have a TDS of ZERO. Of course we can't keep fish in a 0.0 tds water but a lower number does benifit development.
I think the first thing you want to ask yourself is will the fish you own benifit from having a low TDS? Do you have high grade Tosai/Nisai of fish you would consider of show quality then prehaps considering RO might be in order.
As I have stated elsewhere, fish will live just fine in a higher (read: harder) TDS/Hardness enviorment. The most important factors for basic koi health are stocking levels, water changes, filtration capacity, heating, fine partical removal etc. Somewhere down at the end of that list you will also find "low TDS" listed. For the average koi hobbiest I would consider the addition of a RO unit at the very end of my priorities unless I had very deep pockets and a deep desire to expand my learning experiance. In the mean time there are so many other things that one could improve upon that would make a much greater impact on the health and quality of the fish one keeps. I would tend to concentrate on these before I made the leap to a RO water purification system.
B.Scott |