Cheap dark substrate

Hi guys

I am planning on setting up a new 220g planted discus tank and I need to find a cheap alternative to LFS sand/substrate. Before, I always used filter sand or similar, but I always wanted to try a darker substrate. And now is the perfect time to try it, because the tank I bought came with a white-ish 3D background which should prevent the tank being too dark. But now I need help finding cheap dark substrate(black,brown,dark red,...), because the ones offered in the shops would cost me more than the tank did. Curently I found 2 options:

1. lava rock pebbles: easy to buy, cheap, they come in a bit larger size than normal substrate so I am thinking about putting an undergravel jet sistem in. The downside is the are a bit sharp, not the dark red I want and I would have to see it the jet sistem would prevent food from becoming stuck

2. crushed marble: again easy to buy and cheap, it is the colour I want, smaller size(not sure it that would work with the jet). But it also has rough edges and is supposed to interact with the water parameters(I do 10% wc a day so that might cover it)

Any suggestions would be appreciated

Larva rock is not inert, meaning it will break down releasing minerals and increase your water hardness and possibly even raise your pH.
Marble in the other hand is inert and will not effect your water parameters.
In either case a 10% daily water change will supply you with ever increasing nitrates and bacterial counts putting the health of your discus at risk. Any partial water change schedule that does not replace a minimum of 100%or the tank volume weekly is inadequate IMHO :)

Thanks for the input, but are you sure about lava rock and marble? Because it should be the opposite

I am sure, but I have been wrong before, lol. If in doubt use vinegar to test. Apply vinegar to the test subject, if it fizzes even slightly it is not inert.

Garden stores often carry pebbles in 50 LB bags at a fraction of the LFS price. You can get even cheaper deals with more selection at your local quarry. But be sure whatever you choose pass the vinegar test.

Willie

Another good option if you can find it is Black Diamond Blasting Sand - very cheap compared to other substrates and is completely inert.

Thanks for the suggestions guys. I looked into black diamond blasting sand but cant find it locally and for the quarrys, I live in a region that is predominantly limestone so thats an issue. Guess I'll just try to get a few different samples and do the acid test. Luckily I have some strong acids that make the reaction instant and safer

Just an update, lava rock is inert