I want to use something natural, like some real gravel. IS there like a certain rule for gravel or can I use almost anything as long as it doesn't poison the fish or something.

Gravel is gravel. Most of the stuff we use comes from gravel pits. As long as you give it a good wash, it's perfectly okay. Of course you can get your ornamental gravel which I really don't like at all. Gravel can come in different grades, when I've used it in the past, I just get big bags from the garden centre.

Yeah, exactly, I got acouple of stones from the local Rock Store. They a bunch of different grade, types of stones. I even got some milky quartz. But the wife and I like it, we got some dark stones and some tan looking one. Very natural looking.

You have got to be a little bit careful when adding rock or wood to an aquarium. They can have an effect on your pH which you must keep an eye on. You certainly don't want to be adding buffers that have a tendency to lower the pH if you've already got very low pH. Just something to bear in mind.

Some people soak wood or rock in a bucket of water for a couple of weeks just to see what it does to the pH, probably a very good idea, especially if you've got very sensitive fish.

same is true with the coral rock.... it alter the pH
if not treated yet.

so you saying it decreases it??

Crushed coral is used to raise PH

tried to upload pic taken from phone... this did not work.

Ok Retry

So as you can see the rocks aren't your regular out of bag aquarium rocks. Some are a bit sharp. But overall gravel from from the nearest Rock Quarry. I got these samples at the nearest store where they sell these fancy looking rock. The river bed in the Amazon isn't going to be smooth I would believe. Which would make me think that these rocks would be more natural than regular aquarium gravel.

Trying to decide which color to use. Lighter or darker

point of order.
1) those crushed rock will have spaces in-between, susceptible to
dirt and waste accumulation;

2) choosing between the two, i prefer the black one,
3) i suggest you mix it with some smaller pebbles or sand to
lock the spaces in-between.

just my 2 cent opinion.

In conjunction with the question of gravel, which real rocks are safe enough for a aquarium. How about Petrified wood. I know adding that type of gravel and some rocks will raise the water hardness. The area of where I live has very hard water. I hate taking showers here. Anyways, I plan on have a natural looking tank and plan on adding some driftwood. I heard that wood can balance out the hardwater. Is this so.

a dose of salt usually help to soften the water and it
helps to enhance oxygenation. i am not familiar with
petrified wood, but rocks if had been soak with water
for too long will no longer affect the water hardness.
it will help you house good bacteria in the system.
most of the time, only those porous rock could affect
the water due to the presence of some minerals in it.