Another question for the brain trust.
 

OK, this likely simple.

My wife has a 75 gallon aquarium with African Cichlids and some other fresh water fish. The tank is 48.5x18 and 21.5 inches tall. It has rocks and gravel and fake plants so I know it does not have a full 75 gallons of water in it but it does not really matter the precise number of gallons, the rocks just displace some water. Pretend the rocks are water and it is a 75 gallon tank.

We have to do a partial water change periodically and we use a hose with a water bed siphon hooked to a sink to remove and add water so we can't measure the volume of what we remove or add.

My question for the brain trust: The tank is 48.5 x 18 and 21.5 inches tall is 75 gallons if we drop the tank by 10 or 20 gallons how many inches of water is that?

We need a way to figure out an approximate gallons per inch of vertical height. If we want to remove 15 gallons how many inches is that?

75/21.5=~3.5 gal/in

Rough #'s

231 cubic inches per gallon.

Area of the tank is 864 in^2

Volume of one inch of depth is 864in^3

one gallon = 231 in^3

one inch of depth = 3.74 gallons

15 gallons = 4 in of depth

I suspect the tank is a nominal 75 gallons, since the volume works out to 80.4 gallons

3x5 gallon buckets?

Just a thought, aside from your question...I wonder if it would be worth joining an aquarium enthusiast forum for questions you may have - this question and maybe others going forward.

I've had aquariums for 40 years. Fresh and saltwater 125 now�� Do I qualify?

Eh, its a simple math question. If we were getting into bio load, PPM of nitrates/nitrites, the type of lighting to use, what types of ciclids would be appropriate to mix, etc. then yes, head yourself off to monsterfishkeepers.com

But we have enough folks around here that can do simple math :)

Good idea Baz, I was helping a friend refresh the water in his tank, a large one that I had helped sort the plumbing for. And he decided to go for a bigger water change than recommended, Changed almost half the water at one time.
Next morning there were a lot of belly up fish.

Not sure what did them in, too much temp change, too much chlorine added at one time. It's a delicate ballance to maintain in a water change.
I think getting the new water close to the temp in the tank at least is a step to take.
Also some of the chlorine will off gas if left sitting out in an open bucket for a while.

Cheers Richard.

One of the great parts about having a well here in N Florida. None of that to worry about ... just dump the water in, straight from the hose connected to the well pump (so it doesn't go thru the water softener system)

Wait, you dump fish tank water into your well?

Then you drink it? http://forums.pelicanparts.com/suppo...leys/pukey.gif

Or you fill the tank straight from the well?

Fill tank straight from the well. The water that gets siphoned out ends up either on our blueberry bushes or the wifes flowers...

The tank is my wife's hobby, I just "get" to help clean it. ;)

I suggested to her we just mark the level, remove a gallon carefully and measure the difference. She wanted a math solution and I knew where to ask the question.

We also have a koi pond in the back yard. She "lets" me help clean the filters on that as well. The Koi pond has a direct fill from the water well so it is a super simple thing to add water.

Thanks all.

both my pond/pool and the inside tanks use well water
and I do 150% changes often by running a pump out
after draining 75% I just add more water as the pump out removes more

chlorine and chloramine will kill fish and is in city water in many areas
PRIME or other water treatments for fish will remove chlorine but not chloramine
chloramine is very very deadly to fish
use well water if you can
if no well let the water age in a bucket
a few days before it is put in the tank

Wait - as I understood the OP it wasn't possible to measure the quantity removed. If you can measure the amount you remove and you want to remove 15 gallons, just "...remove a gallon carefully" 15 times.

I hope my earlier answer was helpful. As far as I can see it was the only response that actually answered your question.

3.8 gallons per inch.

That is possible in theory, but in practice it would be a royal pain in the ass. Drips on the floor and tedious is not in the plans. ;)

The process of using the hose to vacuum out the old water is super easy and no mess. Adding back is just tap water but it is added back at the temp she said is correct, whatever that is. As I said it is her hobby I just am slave labor. Repayment for many brake bleeding jobs. :cool:

GH, You do not provide what the glass thickness is, however based upon similar aquariums 0.25 inch glass thickness is common. If it is other, no problem simply adjust. You also asked for several data points as well as the math, so to your answer(s) begin with:

Water is 231 cubic inch per US gallon. Further the dimensions should account for the glass thickness on all sides as the water only occupies the inside. So the first equation to use simply finds the inches per gallon in vertical height of your aquarium:

231 cu. in. per gallon / [(18 - 0.25 - 0.25)in. X (48.5 - 0.25 - 0.25)in.] or, 0.275 inch per gallon in vertical height.

If you are after gallon per inch in vertical height simply: 1/0.275 or, 3.6363 gallon per inch in vertical height.

For your data points then:

10 gallon X 0.275 inch per gallon = 2.75 inch (aka 2 3/4")
15 gallon X 0.275 inch per gallon = 4.125 inch (aka 4 1/8")
20 gallon X 0.275 inch per gallon = 5.5 inch (aka 5 1/2")

Now if you want it in metric, you'll need a metric sized tank :D

Yea, we are not nearly critical enough to bother with the thickness of the glass. The margin of error is not that tight for an aquarium. We just need ballpark figures.

Thanks all for the information.