So I finally tested all my tanks' GH and KH today after some frustration with spawning my soft water dwarf cichlids. Results first, and then some discussion.

Methodology

Test kit: recently opened API GH/KH test kit (drop test), expires 2018 and 2020.
Procedure: very clumsily, with each tank tested only once

Results

Tank: A. eremnopyge, 45x30 cm
Substrate: 1.5 to 2 inches of loose peat moss
dGH: 2 / dKH: 0

Tank: A. bitaeniata, 45x30 cm
Substrate: less than half inch Aquael fine quartz sand, extremely decayed Catappa leaves, peat granules, above sand.
Additional remarks: loose peat in filter bag behind HMF filter, used to receive deionised water changes (stopped 3 months ago)
dGH: 3 / dKH: 0

Tank: Crenicichla sp. probably wallacii or closely related, 90x30 cm
Substrate: just under 1 inch ADA Malaya, extremely decayed Catappa leaves, a lot of driftwood
Additional remarks: peat granules in filter bag behind HMF filter, used to receive deionised water changes (stopped 3 months ago)
dGH: 4 / dKH: 0

Tank: T. candidi, 45x30 cm
Substrate: 1 inch thoroughly mixed 50/50 ADA Malaya and ANS bright sand
dGH: 6 / dKH: 3

Tank: A. diplotaenia, 45x30 cm
Substrate: 1 inch loose peat moss capped with 1 inch gardening sand
dGH: 7 / dKH: 3

Tank: A. trifasciata A. and A. sp "Abacaxis", 90x40 cm
Substrate: "burnt earth" (baked clay) above LECA pellets
dGH: 9 / dKH: 5

Discussion

Theoretically, the mechanism for GH reduction is the binding of positive metal ions to sites on the organic molecules, which removes them from the water column. KH reduction is from the addition of organic (tannic, fulvic, humic) acids, which reduce pH and shift the carbonate equilibrium all the way to KH = 0. I'm glad to be able to test these theories out with measurable results using locally available materials.

From my results, it seems that having ADA soil or peat moss or decaying ketapang leaves in contact with the water column does indeed bring dKH readings to 0 and dGH readings to 2-4. These results for aqua soil and ketapang leaves are not surprising. They are proven and popular tools of local soft water fish breeders. Loose peat moss and granule peat moss are also effective, though not as popular locally as overseas.

Watch out, however, that water does flow through these "softening" substances - when trapped beneath sand with little water flow through it, it has little effect (c.f. diplotaenia tank). There must also be enough material for it to work (c.f. T. candidi tank, which probably has too little ADA soil). This was very sad for me - my candidi and diplotaenia have been laying eggs recently, but no fry appeared. I was peat filtering all the water used for water changes, and assumed the parameters would be maintained in the tank - but something must have been pushing the GH and KH up.

A simple and easily available indicator to test your water is hornwort (Ceratophyllum sp.) It melts in all my tanks with soft water (or perhaps low pH), except one (the bitaeniata tank - maybe I made a mistake in my water tests). It grows well in tanks with not so soft water.

Lastly, the trifasciata tank reads high unsurprisingly because the substrate is baked clay made from construction site waste soil (sold as "burnt earth" at local nurseries). I removed quite a few rusty nails, tile shards, and bits of broken concrete while prepping it for the tank. The plants seem to love it though, and the trifasciata spawn profusely there. I have CRS living in a breeder box there too - weirdly.

Hopefully my test results will help AQ friends with decisions about what substrates and water conditioning aids to use for spawning soft water fish. The tried and tested ADA soil with ketapang leaves is definitely a good way to do this. But peat moss (granules or loose) are also effective.

The next thing I'll report back with is whether dGH 3 and dKH 0 is in fact good enough to spawn these soft water fish in. I hope to have good news.

Cheers