Discus

Discus have a reputation for being difficult, and the water is most of the reason. Wild discus come from the soft, acidic tributaries of the Amazon basin: water with almost no mineral content, a pH below 6.5, and TDS that barely registers on a meter. Captive-bred discus are more flexible (many thrive at pH 7.0 and GH 4–8 °dH), but they still need softer water than the average community fish.

The real challenge appears at breeding time. Discus eggs are porous, and hard water deposits calcium on the egg surface, blocking oxygen exchange and killing the embryo. Successful discus breeding typically requires GH 1–3 °dH, KH 0–1, and TDS below 100 ppm. This is close to pure water, and getting it right is what separates discus keepers who breed from those who don't.

For display tanks, a GH of 4 °dH and KH of 3 °dH is a reasonable target: enough mineral content for healthy fish without the extreme softness that makes pH unstable. If breeding is the goal, you'll need to go lower, and at that point the consistency of bottled water blending (or RO) becomes essential.