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Does any sand/mud work if you want that in a tank with talking catfish?
Can I use what I take from a nearby lake?
I live in northern Sweden and the waters here are often clean enough to drink from.
I'm still at the planning stage so so far no mistakes have been made.

Personally, I wouldn't use mud from a local lake. It's not so much the quality of the water, but what might be living in the mud that would concern me. You could end up with some nasty parasites that your catfish wouldn't be able to resist.

I would not suggest using any 'wild-collected' sand, but I do have a sand substrate in all but two of my tanks that house my talking cats.....and it seems to work very well.