I have a set up with akadama as well but I did not use any supplements under the substrate. Tank is set up and going for a few days now -- should I be adding anything if I just use regular tap water? This whole akadama thing is new to me.
Akadama apparently strips so many minerals out of the water upon initial set-up that you just have to keep an eye on it.
In an ideal world the minerals that the Akadama would suck out would be shrimp beneficial minerals (so that those minerals are theoretically available at some point) and having old sea mud under the substrate is probably a beneficial thing with most substrates anyways so that is probably why Liam is suggesting it.
My understanding with a new Akadama tank is simply that you have to keep an eye on it to make sure that it doesn't knock your tank parameters too far out of where you think they are. If you are re-constituting RO water you will have figured out what target water you are aiming for (probably based upon TDS readings) and you will be assuming that that the gH and kH will fall in line based upon your calculations of the RO mineral additive that you are using.
With Akadama in the early stages of a tank those minerals can get sucked out of the water at a significant clip and might make your water TOO soft if you don't keep an eye on it.
This only appears to be a concern or threat during the initial set-up.