No problem, I hope your endlers thrive
. What type of rock? Most rock sold for freshwater aquariums is specifically selected to be inert so that they don't release minerals into the water which may ruin the water for soft water fish. This is why I am specifically suggesting sandstone as it is made mainly from the mineral calcium carbonate will buffer your pH by raising water hardness.
As above, unfortunately it is not as simple as this. Different species fish have different water chemistry requirements depending on where they evolved. Some fish only live in soft water, some fish only live in hard water and some fish are frequently found in both.
If you can grab a pair of tights or stocking from somewhere, these make good cheap filter bags. Doesn't really matter where you put it in the filter, it is going to act as both mechanical and biological media, but your course foam prefilter should stop the biggest particles.
Very basically explained, the ADA AS is releasing acidic compounds into the water column, these acids dissolve the coral (made from calcium carbonate) releasing calcium and carbonate. The calcium and carbonate are what increases the hardness and helps stop (buffers) the pH from dropping despite the acids leeched by the AS.
The following is a good old article that contains some essential reading, it's especially important to understand some water chemistry when you have fish that aren't suited for your tap water:
http://faq.thekrib.com/begin-chem.html
Just ignore that part in the article about phosphate causing algae in planted tanks
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