Kh - Hardness. Do you have any chance to get to the store and get a hardness liquid test (API sells them too) they're about ~$5?
If anything rocks or gravel make soft water hard (like in my case in TN I have no hardness and would need to use dolomite etc to harden it up).
The amount of gravel like I said will IF ANYTHING (and aquarium gravel from the pet chains do NOT do this) if anything they'll make the water harder and with harder water come HIGHER pH (its a chain reactions, I'll spare you the details).
Generally ~2" gravel is needed if you're going for planted tank. Yes this will explain at least your ammonia spike. The beneficial/ needed bacteria is found in 2 places in your tank: The Gravel & The filter. Removing so much gravel can cause this easily, but a few larger w/c will eliminate this (just keep doing w/c and test the water till Ammonia drops back to 0 again).
Prime removes Ammonia too, so this in combo with the w/c will DEF help your tank (on this matter) right now, that's a quick & easy fix, so that's a relief for you there.
I'm sure if anything you're 'by eye sight' adding too much rather then too little, which is no harm really (I do the same, I don't break out the ML syringes for conditioner lol)
So that's all explained and you can easily take care of it over the next days.
What still get's me is HOW your pH from 7.2 goes to 6 IN the tank....Only things like Driftwood, Leave litter or peat will lower your Ph like this inside the tank - There's gotta be SOMETHING inside your tank or filter that lowers the pH and/ or "eats up" the hardenss coming from your tap.
The red grills are more then likely because of the high ammonia (toxic to fish, pretty much like dumping poison in the tank).
I think for the moment, once you get the ammonia controlled, the fish are gonna be OK. I'd not add any new fish meanwhile till we've figure out what's lowering your pH like this.
Do you maybe dose with CO2 or some special substare under the gravel...anything??
I all these yrs with tanks and trying to help in forums, I've gotta admit you're the FIRST to say the pH drops this far in the tank...Usually the issue is low pH from tap get's upped (by rocks etc) in the tank....but we'll figure it all out