This has gotten complicated. What's kH of water? I know there is a lot of chlorine in my taps bc I can smell it if I run them long enough..
So decaying plant matter (from say, my hygrophilia) is beneficial like IAL but it doesn't release tannins?
Yes and no. The right amount of decaying plant matter is good, it produces infusoria. A healthy eco-system can deal with a steady amount of it. If there is too much decaying stuff, then you can run into problems with ammonia spikes.
From the wikipedia entry on KH:
KH - Buffering capacity, temporary or carbonate hardness in the water.
Also known as 'total alkalinity' or 'acid-neutralizing capacity' (ANC) in some countries.
In tanks with a low KH value, say under 2-3d (35.7-53.6ppm), the respiration of the plants or animals at night (CO2 production) can cause large pH shifts which harm and eventually kill aquatic animals. This is called pH shock. It is due to lack of water buffering.
KH is consumed by nitrifying bacteria 24/7 so either your substrate generates more, you perform water changes to supplement it or you add chemicals to add KH to your aquarium as otherwise it causes a pH crash/shock.
If you live with soft water then you may need to raise it before you add aquatic animals to it. Measure the KH value and if it's less than 4d (71.4ppm) then add some of the items below to increase it.
Remember that KH always tends to go down over time (See old tank syndrome). KH is also reduced by the action of nitrifying bacteria and by water surface agitation.