For those who might be interested, I received this thoughtful and thorough response from Neale Monks on the brackish mailing list:
Southdown sand is what in the UK is usually referred to by its
geological name, oolitic sand. The grains are called ooliths, and are
smooth and round but calcareous (they're a kind of limestone). The
ooliths are calcium carbonate that has precipitated out around tiny
pieces of fossil shell, and in some places the ooliths are cemented
together into limestone. Anyway, no idea why this is the substrate _du
jour_ for marine fishkeepers, there's really nothing special about it.
If its easy to get and cheap, then by all means use it. I'd imagine the
eel will enjoy burrowing in it compared with something rough like coral
sand.
My only caution would be that oolitic sand is calcareous and will
slowly elevate the pH and hardness. I like the mix of substrates you
are using now... I wouldn't push the pH too high with a fire eel. They
aren't euryhaline fishes, and a pH of 8 and an SG of 1.008 would have
to be pretty near the upper limits for the fish. In the wild it is a
neutral freshwater denizen, and tolerates rather than requires salt.
Obviously it is doing fine, and given that they aren't particularly
hardy or easy fish to keep I wouldn't do anything to change the
conditions you have now. Since the monos and archers would be perfectly
happy in marine conditions, you have a mixture in the tank of fishes
from the salty end of brackish and fish from the fresher end. Really, I
think you have struck the happy medium and would be wise to stay there,
so replace your current substrate with caution, and keep an eye on pH
and hardness.
The standard advice is to use gravel to a depth of around 8 cm or so
with an undergravel filter, and less than 2 without one. Complicating
this somewhat is the size of the grains in whatever substrate you
choose: coarse pebbles are less likely to create anoxic spots than
silica sand. But there are other factors to consider. If you use
plants, a healthy root system will prevent anoxia in the substrate
amazingly effectively, and I've used silica sand and laterite up to 8
cm depth with plants without an undergravel. Similarly, if your fish
burrow or otherwise move the sand about, then again, you can be more
liberal about how much sand you use. Use a depth that is either enough
for them to "earth eat" if that's what they do, or cover themselves in,
if they do that instead.
A filter is a filter is a filter (in my opinion) and the difference is
between people who choose the appropriate size and design for their
aquarium and those who don't. Your local water supply is also an issue.
If you have low nitrates (rare these days) the regular water changes
are the best strategy for water management. On the other hand, if your
local nitrates and phosphates are high right out of the tap (faucet),
then you may also need to consider that when designing and installing a
filter. Frankly, none your fish are especially sensitive to nitrates,
and so unless you have something like 80 or 100 ppm in the aquarium,
I'd just concentrate on something that handles the ammonium / nitrite
removal. I've found nitrate removal to be an expensive and not very
effective process, and prefer to simply use reverse osmosis or
de-ionised water for changes where low nitrates are required. My local
supermarket sells such water for about 60 cents a gallon, and that's
fine for the Triops aquarium I have sitting on the window sill in my
apartment. I'm not convinced a DSB, or "deep sand bed", is an option
given your set-up. There are too many big active or predatory fish.
Nitrate removers work much better in low stocking density aquaria where
bacterial nitrate removal is augmented with nitrate removal by algae as
well.
Sand doesn't go into suspension, silt does. If the sand is cleaned
well, any residual turbidity once introduced to the aquarium will
settle out quickly, certainly within a few days. The fish don't care.
Anyway, in summary:
1. I'd be cautious with oolitic substrates, and given your assortment
of fish would either stick with what you have or else opt for something
smoother and finer but still neutral chemically, like silica sand or
pea gravel. It is far easier to add hardness and pH increasers to the
water or filter than it is to decrease hardness and pH.
2. Use whatever filter works for you.
3. The sand won't add to the turbidity of the water in the long term.
Cheers,
Neale
So, I'll stick with my pea gravel and try to find a like-sized coral sand to mix with it. Thanks, all!
