and Calculating Volumes
contains information and advice on how to do
some measurements and calculations that you
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1. How much is 20% of my Aquarium's Water?
here to read about changing 20% of the water
in your aquarium or pond. How do you calculate
20%? Here are three examples.
If you have a 10-gallon aquarium. Calculate
20% of 10-gallons = 0.20 x 10-gallons = 2 gallons.
Multiply the 2 gallons times 16 (cups/gallon),
to get 2 x 16 = 32 cups. Use a clean measuring
cup to remove 32 cups of water from your aquarium.
Now mark the level of water in your aquarium
with a permanent marking pen or a piece of tape.
In the future, drain the water in the aquarium
down to the mark and replace it with fresh tap
You know your pond holds 50-gallon of water.
Calculate 20% of 50-gallons = 0.20 x 50-gallons
= 10-gallons. Use a clean bucket with a mark
at one gallon to remove 10-gallons from your
pond. Put a clean stick in a certain spot in
your pond and carefully mark the stick at the
water line. In the future put the stick back
in the same spot in the pond and pump the water
from your pond down to the mark on the stick
then refill the pond with fresh tap water.
You don't know how many gallons of water your
aquarium holds, but you can see that your aquarium
has vertical sides. That means straight up and
down. Measure the water depth and multiply by
20%. For example many small aquariums are about
12 inches tall with water about 10 inches deep.
Then 20% of 10 inches is 0.20 x 10 inches
= 2 inches. Measure 2" below the surface
of the water and use a permanent marker to put
a mark on the aquarium glass. You still don't
know how many gallons of water your aquarium
holds, but you do know how far to drain the
water to replace approximately 20%.
Remember that removing 20% of the water and
replacing it with tap water from the faucet
is usually safe even without water conditioner,
but replacing more than 20% can be risky even
with water conditioner. So twice a week remove
20% of the water and replace it with tap water.
2. How much Water Does my Aquarium Hold?
here to read the Recommended Treatment,
which includes adding one drop of Quick Cure
for each gallon of water and one Tablespoon
of Aquarium Salt for each five gallons of water.
To give your fish the Recommended Treatment
you'll need to know how many gallons of water
your aquarium holds. Here's how to measure and
calculate the volume of your aquarium.
Measure your aquarium from the left
edge to the right edge. For example,
my aquarium measures 18 inches.
Measure your aquarium from the bottom
edge to the top edge. My aquarium
is 12 inches.
Measure the aquarium along the side
from the front edge to the back
edge. Mine is 12 inches.
Multiply the three measurements.
I got my small calculator and carefully
entered 18 x 12 x 12 = 2592 cubic
FINALLY multiply that number by
0.00433 to convert from cubic inches
to gallons. I got 11.2 gallons of
water in my aquarium.
To get an even more accurate answer I realized
I should have measured inside the aquarium.
Measuring inside I get 17.75 x 11.75 x 11.75
= 2450.6, multiplied by 0.00433 is 10.6 gallons.
So measuring inside the aquarium instead of
outside reduces the volume from 11.2 to 10.6
gallons of water inside my aquarium.
3. A Slight Correction
But my aquarium also has a big rock and some
gravel that reduce the amount of water in my
aquarium. I'm not sure of the rock's volume
in gallons, but I measured it by doing the following
a big clean bucket.
the rock in the bucket.
enough water to cover the rock.
assistant held a finger just touching
a measuring cup to add water to
Until the water just touched my
total was 38 oz. added to the bucket.
Divided by 128 (oz./gallon) to get
So this measurement shows the rock's volume
is 0.3 gallons. I was surprised by this answer.
I thought the rock had more volume like maybe
a gallon, so I repeated the measurement, and
the second time I put my finger just above the
waterline while my assistant added the water.
We got 37 oz. or 37/128 = 0.30-gallons. We decided
0.3 gallons is close enough and my estimate
of about a gallon was wrong.
I realized that the rock had a hollow cave inside
and lots of other small holes. So it had fooled
me into thinking it had more volume than it
did. I took note of my mistake, so that next
time I'd take the fact that a rock or something
else is partially hollow into my estimate.
For more accuracy
we subtracted the 0.3 gallons for the water
displaced by the rock from the 10.6 gallons
to get 10.3 gallons of water in my aquarium.
There is still the gravel in my aquarium with
an estimated thickness of 1/4-inch. The bottom
of the aquarium measured inside is 11.75 inches
times 17.75 inches = 208.6 square inches. At
1/4 = 0.25 inches thick or 208.6 x 025 = 52.15
cubic inches of gravel on the bottom of my aquarium.
Next multiply this number by the same conversion
factor, 0.00433, to convert the answer to gallons:
52.15 x 0.00433 which is about 0.23 gallons.
But the layer of gravel is not solid gravel,
it is, say, 50% rock and 50% water.
So take half of 0.23 gallons to get 0.11 gallons.
Of course, this estimate of the gravel might
be a little inaccurate. I'd say it might be
half as much or twice as much. So it's likely
between 0.05 and 0.20-gallons.
I could put all the gravel in the bucket and
measure its volume just like I measured the
volume of the rock. But for now I'll use the
calculated value of 0.11 and subtract the 0.11
gallons from the 10.3 gallons to get about 10.2
gallons. Not much of a difference, so I can
probably ignore the inaccuracy in the volume
of the gravel.
There's about a pint of water in the filter
that hangs on the back of my aquarium. A pint
is 1/8 of a gallon, or about 0.13 gallon. Add
that to the 10.2 gallons to get back to 10.3
gallons. I think this number is pretty close.
The first easy measurement and calculation gave
11.2 gallons of water in the aquarium. A more
refined measurement and calculation, that took
a lot more time, gave 10.3 gallons. So I'd put
10 drops of Quick Cure instead of 11 drops.
This is one drop less and probably wouldn't
matter anyway. But it's good exercise to make
these calculations and to find out that the
original number 11.2 was probably close enough.
The more accurate number, 10.3 gallons, is more
accurate than we needed.
We could refine
our methods and get a more accurate answer,
maybe something like 10.27 gallons. But this
is more accuracy than we need. We're just trying
to find the answer to the nearest gallon here,
so we can add one drop of Quick Cure to each
gallon of water. The point here is that we can
be very sure the volume is between 10 and 11
gallons, and is probably closer to 10 than 11.
If you make these kinds of measurements and
calculations for a few years, you will develop
a feel for what matters and what doesn't. But
even after 30 years of doing these calculations,
every so often my confidence wavers, and then
I make a calculation and a measurement just
to see if the two numbers are about the same.
If they aren't the same, I think about it, make
some more calculations, and some more measurements.
The dimensions you measure on your aquarium
will probably be different from the ones that
I measured on my aquarium, but the formula is
the same. Here is the formula for any normal
V = L x W x D x 0.00433
where V is the symbol for the Volume in gallons,
L for the Length, W for the Width, D for the
Depth, and the 0.00433 is a so-called conversion
factor that converts the answer from cubic inches
Maybe this is more than you wanted to know about
aquarium arithmetic. Notice that we measured
some things and used those measurements to calculate
other things. Measuring and calculating is fun
and good exercise for the brain once in awhile.
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