The idea that leaves with soften water significantly is a little misleading. If you add lots (bushel baskets full to a 6ft tank), it may reduce pH slightly. But the main beneficial result is the adding of tannins, which have a slight anti-bacterial effect. Whether this effect is needed by Uruguayan fish is debatable, compared to Amazonian species (3000 miles to the north)
The leaves do breakdown after a while, turning to detritus which can then have the opposite effect, of adding debris which can be a great substrate for bacteria (and not the beneficial kind) to proliferate. And as they break down, they also tend to add small suspended particulate, which can be "not" anesthetically pleasing.
I experimented with leaves in 20 gal killifish tanks, and found the pH might drop a 10th or so, when I added a layer leaves perhaps 3-4" deep. After a while the leaves broke down to dust and needed to be vacuumed out. The killifish used the leaves as a spawning medium, and was a great place for fry to escape predation.
Because cichlids have a tendency of ripping leaves to shreds, if you want the benefit of tannins in a cichlid tank, it may be better to suspend them, or peat in a bag in a sump, easily removed when becoming problematic.
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although they start as above, they soon end up like this (especially in a cichlid tank).


young dimerus below

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