These wonderful images of Angelfish with
fry testify to the safety of slate in an aquarium. I especially
enjoy the parts where the fry can be seen nibbling on the
rough edges of the slate. I suspect that tiny algae
plants, too small for us to see, are their target.
"Slate killed my fish"....."
Slate made my fish sick"..... Neither of those
internet searches turned up a result to question the
prevailing wisdom that slate is a safe stone to use in
aquariums. That said, my experience in working with slate
and other types of natural stone tells me some caution and
a close inspection of the stone is a good idea. Some
slate, especially slate that has been quarried from a deep
quarry and quickly packaged for sale, will leach minerals
into water. This leaching can be very obvious with
pigments colouring the water, or it can be
Stone that you find on a beach will have
long since leached out it's soluble minerals. Stone that
is freshly quarried or has been kept mainly dry since it
was quarried should be soaked for several days. The water
should be changed occasionally over the soaking period.
This simple procedure will make most slate safe for fish.
Another precaution is to check for some visible features
of the slate that might indicate potential problems.
Avoid using slate that has a soft dusty surface. Scraping
your fingernail across the stone should not leave a mark,
unless it's some fingernail residue. There should not be
any accumulation of stone dust under your nail.
Don't use slate with cavities or areas
where the hard stone flakes away to expose dusty or
crystal like surfaces. Cavities (most easily seen on sawn
edges) are more likely to contain mineral concentrations.
In particular, do not use slate that has sparkly gold or
silver crystals. These may look pretty but are probably
pyrites or galena. Minerals that contain lead which will
Slate occurs in a whole range of colors,
including some extravagantly multicoured versions. I have
used some multicoloured slate in my aquariums without any
sign of problems. I have also read suggestions that only
solid one colour slate be used in a fish tank. This
caution is probably unnecessary but is understandable
since nobody wants to risk poisoning their fish.
Surface discoloration can be a result of minerals leaching
from the stone and could be the cause of fear of colours.
Multicoloured slate can contain more cavities than solid
colour slate and is somewhat more risky from that point of
view. I believe that colours that are truly in the stone
do not present a problem while colours on the surface only
are a warning sign.
Aquarium supply retailers can usually
guarantee the fish safety of their stone. There are also
on line suppliers who deliver guaranteed fish safe stone.
The other danger that is sometimes mentioned with regard
to slate in an aquarium is the risk of fish being harmed
by sharp edges. Certainly freshly broken slate can have
very sharp edges. It can break sharp enough to easily cut
a hand, so no doubt could also cut a fish. Fortunately the
sharp edges on slate can be very easily dulled. Simply
drag an old knife or similar metal tool along the sharp
edges and they will very quickly become blunt.