I did the usual water change regime this morning. The catappa leaves were due to be replaced and, after boiling the new ones to soften them, I decided to add the cooled-down, tannin-stained water along with the dechlorinated water into the tank, to give the fish a more authentic environment which I thought they would like. However, they don't seem to be too happy - 7 hours post- water change and the tetras in particular are still crowded together in one location, their tails not yet back to their usual bright red, while the harlequins are much more stationary and schooling more than usual.
The saucepan should have been fine ie no remnants of food or washing-up liquid in it, so I don't think that can be to blame. I'm wondering if the fish will just get used to the darker water in time or whether I ought to do another water change in order to restore some normality for them (or might that stress them further)? Another water change would run the risk of the already-low KH (1.5 from the tap, 3 in the tank with the limestone rock) becoming even lower. The fish were perfectly fine prior to this water change - in fact, I'd just been commenting how great it was that they were all doing so well and that the QT hadn't been in use for more than 6 months since the cories' problems. Thoughts welcome; thanks.