Planted Tank Enthusiast
Join Date: Jul 2014
Location: California for school, Arkansas for home
I don't know that much about Discus...but from what I've read:
Depending on where in the Philippines you live...I'd recommend using a mixture of rain water and fallen brown talisay (Catappa terminalia) leaves. The leaves bring down the pH of the water (you'd have to check by how much) by releasing humic and tannic acids. You then mix that water with your regular water to bring down the pH of your regular water. You'd have to make sure the rain water wasn't polluted (ie you don't live in a super industrial area), and that the talisay leaves aren't sprayed with pesticides).
The substrate you mention buffers(brings the water parameters to) the water to a preferential range, which you also have to figure out. For example, aragonite sand substrate preferentially buffers the pH to approx 8.2 . The same applies to your substrate, but I have no experience with Onyx or ADA Amazonia, so you'd have to check, again by testing.
But hey! Why are you doing discus? If this is absolutely something you want to do, then go for it! But I'll caution you by saying this: discus are very, very expensive in upkeepand depending on where you get them from, fragile and picky. You might want to check and see if there isn't some other fish that would do better with your water. Surely your local fish store, retailer, or website (wherever you buy your fish) can suggest fish that would like your water, allowing you to do more water changes and keep them healthier!
So many fish to keep, not enough aquaria.