thank you for your replies.
Somehow I think though that none of them applies, – the one regarding rotting leaves and decomposing things could actually be relevant, – because I do have lots of leaves, and organic materials, – but I know for a fact that with the four linkeis I changed everything in the tank, – its was all new, and theres no old material in that tank.
And it has been for a long time with the malpuluttaes, and they dont ‘deteriorate’ – at all. They seem to live fine with it.
However, I will look into this with the decomposing stuff, because it sound interesting. I am using quite a bit of peat and maybe the quality of is not perfect, or could cause something if left too long.