40ppm is quite high - you would not want your nitrates to sit at this level.
Yes, you can baffle your filter output, or use an air powered sponge filter. You want filtration with minimal water flow.
It's good to hear you already pay a lot of attention to hand washing!
No, all types of sand are not suitable for axolotls
this Sanctuary article is full of excellent information! As I said above, I'm really not one to comment around the chemistry of coal slag, so perhaps you are able to determine if it fits within one of the listed safe sands - someone else may step in here. It is worth searching the site and you can judge from the other opinions given in the past
One very good way to think about it is - would axolotls live on coal slag in the wild?
Please understand that we are not here to contradict what you already know about your fish. We are here speaking from experience. Lots and lots of experience - specifically with axolotls and other amphibians. And we have also seen lots and lots of axolotls get sick and die due to poor or inappropriate husbandry. Axolotls are a dream to keep if you follow the guidelines given on this board. Of course, you are
always free to extend your boundaries and use your prior experience with aquariums to make educated decisions on how you feel a habitat would be better be formed for an axolotl!
There is a whole
forum section on tanks, setup and substrate for hours of perusing pleasure! Hopefully we'll see some of your pictures up there soon! Hopefully with a smiling husband in shot