My aquarium water turned black!!

About 3 days ago I set up a nano 5litre aquarium just as something to do with some left over plants and wood I had. It was looking great for the first day but then the water started turning black. I first thought it could be tannins but by the end of day 2 it was definitely black water and not tannin water. I did a 100% water change on day 2. Water was again clear, then this morning it was completely black again. I have just done a 50% water change and don't really know what to do from here because I don't know what's causing the problem.

The tank has no heater and no filter, I'm not planning to add any livestock to this tank so didn't bother putting in any equipment. I have been using prime in the water as well as some excel and also some stability out of habit for when I set up new tanks.

This is the tank after 100% water change

A photo after 50% water change

I forgot to take a photo before a water change but it was like this
Possibly even blacker.

The water itself is clean/not cloudy but just black. I couldnt even see the driftwood or plants. A Google search hasn't helped as the posts have described factors from their aquarium that are not the same or applicable to mine. I have never had this happen before and don't know where to start to fix! Any advice is much appreciated !

Why not remove the wood for a day or two to rule out tannins?

seangee said:

Why not remove the wood for a day or two to rule out tannins?

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That's a good idea/point, I've just never seen tannins that turn the water black instead of tea coloured particularly with spider wood I just added a bag of purigen to the tank to see if that helps/ if it is tannins

Wow, not something I’ve seen before.

It’s not tannins I believe, I’ve had tannins in tanks before as well as Rooibos tea stained water and none of them were this colour at all, most I got was tea coloured water which I like.

Believe either wood or stones/rock is leaching something into the water, no idea what though.
Could even be the sealant or tank itself somewhere.

Very hard to tell from just photos.

Sorry that’s not particularly helpful but am at a loss to explain this.

Agree to take the wood out for a wee while to see if that makes any difference.

If no difference after a day or two then put wood back then try removing some of the stones/rocks to see if that helps.

A case of step by step to eliminate possible causes until eventually finding the culprit hopefully.

Is there no filter or pump to create a flow in the tank to turnover the water?
A very slight possibility that may help.

Very interesting, I’ll be keeping an eye on this thread to see what happens next.

Ch4rlie said:

Wow, not something I’ve seen before.

It’s not tannins I believe, I’ve had tannins in tanks before as well as Rooibos tea stained water and none of them were this colour at all, most I got was tea coloured water which I like.

Believe either wood or stones/rock is leaching something into the water, no idea what though.
Could even be the sealant or tank itself somewhere.

Very hard to tell from just photos.

Sorry that’s not particularly helpful but am at a loss to explain this.

Agree to take the wood out for a wee while to see if that makes any difference.

If no difference after a day or two then put wood back then try removing some of the stones/rocks to see if that helps.

A case of step by step to eliminate possible causes until eventually finding the culprit hopefully.

Is there no filter or pump to create a flow in the tank to turnover the water?
A very slight possibility that may help.

Very interesting, I’ll be keeping an eye on this thread to see what happens next.

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Thanks for your input! I've got another tank that's exactly the same from the same seller which I've just added water to, to see if it may be the aquarium seals. If not I'll begin the process of elimination with the tank in question. I'm also at a loss of what could be causing the black water, especially as I've used the substrate from the same bag in another aquarium, as with the wood and rocks. I'll be sure to update if I can find out what's going on

Hmm, same seller? An LFS or private sale?

Sorry if this is an obvious question but you wash the substrate before adding to the tank?

Are you using a filter in the tank? If so, is there carbon in the filter?

Ch4rlie said:

Hmm, same seller? An LFS or private sale?

Sorry if this is an obvious question but you wash the substrate before adding to the tank?

Click to expand...

It was from a private seller. But the 2nd tank hasn't had the same happen so I've removed the driftwood for now and placed it into a bucket and did another 100% wc for now. And I do wash my gravel and substrate too!

EllRog said:

Are you using a filter in the tank? If so, is there carbon in the filter?

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No filter in the tank. I'm not adding livestock so didn't add one

Its the wood. Looks cured,but only the wood would cause black water. On the outside it could be bacterial?..then you could have tried that "gray water cure" they sell. I hear it really works.