If you have access to a steady supply of clean (safe) rainwater, I would use it alone and not add any tap water. The lower the GH/KH/pH the better for fish like otos. Mine spawned (on their own, didn't know they had until I counted five instead of three one day, with two being a tad smaller than the original three) in zero GH/KH with a pH below 5. I am fortunate tohave tap water of zero GH/KH.
If the rainwater is not guaranteed year-round, moving to RO would be preferable. Pure RO, no mixing of tap water. This will give you zero GH/KH and the pH will naturally lower accordingly.
On the safety of the rainwater, the GH and KH that high puzzles me; rain water as it begins to fall is zero GH/KH and a neutral (7.0) pH. It changes only by assimilating substances, since water is the strongest solvent on the earth, readily assimilating whatever substances it comes into contact with, such as mineral from rock, organics, etc.
Thank you, this was helpful!
In an ideal world, I'd only keep fish that thrive in hard water, given my beginner status and limited funds/access to RO. But I was recommended otocinclus to keep in my tank with guppies, got some, fell in love with them and enjoyed them for nine months, then came here and found out that my water is too hard for them.
Seangee and Essjay were teaching me about the effects of hardwater on softwater fish, while others were telling me to just get rid of them. Given how delicate otos are and how often they're bought in ones and twos as cleaner fish and not given the right conditions, Sean and I agreed that keeping them where they were thriving, albeit in the wrong water, was better than returning them to be sold on, especially since the damage to them has likely already been done
Since I'm keeping them, I wanted to set them up in a softer water tank, and have got a tank and things ready, but paused it while the current tanks are in treatment so I don't spread worms or eggs to a new set up. Essjay said that theoretically, rain water should be chemically close to RO water, so the plan was for me to use a mix of rain and tap water, until I get my hands on RO. I hoped if they were the same, I could use mostly rainwater, and resort to RO only when that's unavailable, making things much more affordable and manageable for me.
The reason for wanting to use a mix of either rainwater or RO with tapwater is cost and availability. I have access to some clean safe rainwater (covered plastic rain barrel that comes from a covered plastic gutter, that my dad used in his tank for years without ill effect) and the nearest RO place to me is a 30 minute drive. Buying and storing enough RO to use only RO, or buying an RO unit, would cost a lot more and be a lot more difficult, and funds are tight
So I anted to get the hardness down enough to be okay for them, even if it was at the upper end of their range.
Hmmm, I didn't know why there was a reading for KH and GH for the rainwater either, I figured I just didn't understand properly, or that perhaps the test strips I had used were inaccurate, and planned to test the water from each source again when i had better testers. But a thought just occurred to me... my dad would sometimes top up the rain barrel from the hose pipe when there had been in a dry spell, and leave it uncovered "to let the chlorine gas off", since he distrusts tap water and water conditioners. He may well have done that at some point without my knowing, and the lower part of the barrel below the tap would keep some water for a long time, only gradually diluted out as the additional rain water collects, mixes and drains. Perhaps there was enough tap water remaining in there to give a reading?
I can empty the barrel and clean it out then retest. The gutters are being cleaned soon too, so that would be a good time to do it.