Help with substrate

I bought a 2nd hand tank, came with gravel and then this black substrate.
Was told it was for aquascaping & good for plants.
Anyone know what this substrate is?
Also should the water be that black when it's wet?
Or does it need a good good clean?

Normally the black stuff is “capped” by adding layers on top of it to keep all of that down below for plants’ roots. Sometimes powdered clay is added on top and then gravel on top of that, other times just gravel. It is up to you as to whether or not you want to keep it. It may be beneficial if you’re doing a lot of stem plants that are higher-tech or ones that require some CO2. That soil will eventually deplete and need to be replaced. If it was me, I might use some in a tiny plant only aquarium but I probably wouldn’t put it in one of my larger aquariums with fish. The thought of needing to change it out within a year (or two, or three) just sucks the life right out of me. You need to be careful when vacuuming substrate as you don’t want to get in too far and suck up all of your soil and then end up with a muddy water column.

I would never use second-hand substrate, regardless of what it is.

The black substrate is aquarium soil.
1. This stuff is delicate you cannot pre rinse or wash it otherwise you will just get a muddy mess as you break down the structure.
2. You have to place it in the tank delicately then fill the tank slowly with water as to not to disturb the water. You will still get a lot of "dust" floating through the water but it clears up in a few days.
3. You do not need to cap it. Some do but to my way of thinking it makes it even more difficult to deal with.
4. As others mentioned it has a life, you cannot clean and re-use. You will have to replace it at some time in the next months or years.
5. I have examined the Tropica version of this under the microscope and there are sharp fragments in the matrix. I wouldn't use with cory's unless it is capped.

I second @Slaphppy7 comment. If the material was already used throw it out, its not like gravel or sand that can be cleaned and sterilized. But if it is still new it does make the plants grow real nice for a year or two.

Dump it if it's used. Sand is all you need to get healthy plants.

I would get rid of it. All so-called "plant" substrates have serious risks for substrate fish especially. Capping it with sand makes no difference, it is a bacterial issue.

The best overall substrate is soft sand. All plants grow well in it, you can use substrate fertilizer tabs if needed for certain plants, and sand is safe for almost all fish (some very large cichlids may have issues). A quality play sand is so inexpensive. You are in the UK so look for Argos Play Sand, many UK members have posted good things about this sand. Or you can use an inert aquarium sand which will be very expensive, but safe (if it is iner)(.

If you don't have an established filter for that tank, the substrate could benefit you. Of course, if you get a bit of filter media from another tank, that could help you cycle the new tank without dealing with the plant substrate.