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I have two "twin" 10 gallon tanks that I have been using for comparative
experiments for a long time. A couple (or maybe three) years ago I
decided to use them for a long-term substrate experiment. This rather
long letter summarizes the result of that experiment.
In one tank I used Dan Q's kitty litter and osmocote substrate. I used a
washed mixture of Tex-blast, coarse sealed aquarium gravel and a small
amount of very fine sand, all from a previous setup. The lower 1 1/2 inch
of the substrate consisted of that mix with about 10% by volume plain
kitty litter. I sprinkled general-purpose Osmocote pellets over the top
of the layer and covered that with another inch of the plain gravel mix.
In the second tank I used a variant on a soil substrate. The lower 1 1/2
inch was sandy sediment from the nearby river. It was mostly fine sand
with a small amount of silt and clay. The top inch was the same plain
gravel mix that I used in the first aquarium.
Both tanks had an undergravel filter plate under the substrate. The UGF
was not used and the tanks weren't otherwise filtered. I covered the
plate in the second tank with some of my wife's old nylon stockings to
keep the fine river sediment from filtering through the UGF plate. I just
slid the plates into the nylons, then cut them to length.
Both tanks were initially populated with the same mix of plants (C.
wendtii, Val. americana, Anubias barteri nana, Egeria densa). Initially I
got better growth from the osmocote and kitty litter substrate. Growth in
the tank with the river sediment substrate caught up after a few months
and through the rest of the period the river sediment tank produced more
and better growth than the osmocote and kitty litter substrate.
The test wasn't completely static. I changed the plants in the tanks as
time went by, and the plant populations themselves changes as some plants
burgeoned and others failed. I changed the lighting, the methods of
fertilizing the plants, and I added CO2. Every change was made to both
tanks at the same time, so there was never much difference in the way the
tanks were maintained. I even kept the fish, snail and shrimp populations
as similar as I could.
Several weeks ago I pulled a large C. wendtii from the river sediment tank
because it was blocking the pump outlet. That unleashed a huge cloud of
mulm that took a long time to settle out. When it did settle it settled
on top of everything and even after weeks of cleaning it was still there.
That was followed by an outbreak of both beard algae and a type of green
hair algae. Then blue green algae reared it's ugly head.
Today I decided to take those tanks down, look things over and summarize
the results. Both tanks had problems over the period.
The osmocote/kitty litter tank grew C. wendtii relatively poorly, but
supported a huge stand of vals. At the same time the river sediment tank
grew a big stand of C. wendtii and the vals went into severe iron
The huge growth of vals in the osmocote/kitty litter tank was followed by
a bluegreen algae outbreak. I beat the bluegreen algae and spurred a
burst in the C. wendtii growth by clearing most of the vals, adding E.
tenellus and DIY CO2. In the river sediment tank I fixed the iron
deficiency with iron gluconate tablets, and got in addition a huge burst
in growth from a previously struggling C. retrospiralis.
More recently, I used both tanks to grow out a lot of young Barclaya
longifolia. Both tanks produced good growth, but on removing the plants
today it appears that I got better tuber development in the river sediment
It appears that the very little of the mulm settled through the river
sediment; most of it all packed up in the top layer of the substrate.
The vals, barclaya and C. retrospiralis all kept their roots mostly in the
layer above the river sediment. The C. wendtii sent it's roots mostly
straight down and spread them out under the river sediment inside of the
nylon stocking on both sides of the UGF plate.
By contrast, roots of all plants in the kitty litter/osmocote tank seemed
to spread through most of the thickness of the substrate. C. wendtii in
the kitty litter/osmocote tank had its roots through the UGF plate, but
not anything like the mess in the river sediment tank. When I pulled C.
wendtii out of the river sediment tank the roots were badly broken. When
I pulled the C. wendtii out of the kitty litter/osmocote tank the roots
were in much better shape.
The mulm produced in the river sediment tank was very light and fine, and
there was a huge amount of it. The mulm in the kitty litter/osmocote tank
was coarser, settled more easily and there wasn't as much of it.
While the river sediment tank did produce better growth than I got in the
kitty litter/osmocote tank, the difference was not large enough to offset
the final instability in the river sediment tank. I'll never again build
a substrate with a layer of such fine material.