When a 55 Gallon Aquarium is not a 55 Gallon Aquarium

When a 55 Gallon Aquarium is not a 55 Gallon Aquarium

  • Author bsimon615
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When is a 55 gallon aquarium not a 55 gallon aquarium?

Or any other aquarium for that matter not exactly as advertised. The size of any aquarium is based upon it’s dimensions and measurements would indicate that virtually all aquariums do not have the water holding capacity that you would think they have based upon how they are labeled and marketed. As long as this is understood, no problem. Adjustments may have to be made when dosing medications or in determining when the aquarium is at capacity for fish or in determining what filter may work best. True water holding capacity will be important when you are doing your water changes. A 20% water change may actually be a bit more because any size tank will in virtually all cases hold a bit less water than you think as volume calculations should be based upon inside dimensions and also factoring in aquarium contents including gravel, rock work, driftwood and decorations. Accordingly a 20% water change may actually be closer to 25% in some cases.

So how are aquarium sizes, but maybe not water holding capacity calculated? Basically it is easy to determine with careful measurement. Taking outside measurements, (the measurements usually found on the protective cardboard wrap or box that the aquarium comes in or the removable label on the aquarium glass) will give one an inflated figure of aquarium capacity, often very inflated. For example using the measurements provided for an Aqueon 55 gallon aquarium which are 48.25” x 12.75” x 21” will, if used (multiplied), give you a volume of 12918.9 cubic inches. Dividing this figure by 231 (the number of cubic inches to a gallon) will give one a measurement of 55.9 gallons – so a little something extra. But wait, measuring from the outside dimensions does not tell someone what a 55 gallon aquarium holds within it’s sides. As those sides have thickness and as the bottom of an aquarium is typically lifted above the extreme bottom of the bottom frame and as the water does not fill the aquarium to the extreme top of the top frame, we have to recalculate taking all of this into consideration..

So adjusting the numbers for a 55 gallon aquarium let’s subtract ½ inch for the length as the glass is typically 5 mm to 6 mm thick all around, so the inside length becomes 47.75 inches. Same subtraction for the width, so that becomes 12.25 inches. The height subtraction is even more dramatic as in the Aqueon aquarium line for tanks ranging from 15 gallon to 55 gallon the bottom glass will sit about ½ inch above the bottom of the plastic frame and as the glass is typically about 6 mm thick for the 55 gallon aquarium we need to subtract .75 inches from the height for the bottom and an additional 1 inch subtraction due to the reduction in height of the water surface below the top of the top frame. The total height reduction then is 1.75 inches, so the adjusted height for measuring volume becomes 19.25 inches. So here we go 47.75 x 12.25 x 19.25 which gives us a volume of 11,260 cubic inches which when divided by 231 yields a gallon capacity of just about 48.75 gallons. So in truth, the volume capacity of a 55 gallon aquarium is in reality a bit less than 89% of advertised capacity. And this is the water volume capacity before substrate, rocks, driftwood, etc., etc. is factored in. By the time all of that is calculated the 55 gallon tank’s capacity may drop by another 4 to 6 gallons if we have a densely packed pea gravel substrate with little water holding capacity coupled with a good amount of rock work and driftwood. So maybe now that 55 gallon aquarium is down to an anemic 42 to 44 gallons of water.

Note that larger gravel has a higher water holding capacity in that if let’s say you carefully measure water and begin pouring it into an empty aquarium with a two inch depth of gravel with maybe each piece of gravel approximating the size of a green pea or a kernel of corn, it will hold a surprising amount of water before the water level begins to rise above the level of the gravel due to the gaps between individual pieces. Finer grained pea gravel or beach sand or play box sand will not hold nearly as much water before the water level rises above the substrate level as each small piece or grain is more tightly packed. Other items like large rocks, small boulders and driftwood have to be calculated in by using other tactics. Essentially a rise in water height has to be calculated as these items are added. If these items added to a 55 gallon aquarium result in a one inch rise in water height, your water displacement is 2.5 gallons.

Some capacity can be added by calculating in the water volume contained in your canister filter and/or hang on the back power filter. This for moderate size aquariums will rarely be more that 1 gallon and typically less than ½ gallon for many filters for these sized tanks.

Does it all really matter? For many it won’t. Aquarium selection is based more on available space, budget and aesthetics than aquarium volume. However knowing the true water volume may need to be taken into consideration for medication dosing and filter selection as well as for tank population. Perhaps most importantly knowing true water capacity is key when calculating just how much water is being replaced when you do your water changes.

The 55 gallon tank is relatively close to advertised size if only the outside dimensions are considered in calculating that as using those figures yields a calculated volume of 55.9 gallons. However, as previously explained, inside dimensions are what should be used when calculating water volume. Using those dimensions the water volume capacity drops to 48.75 gallons. Other Aqueon tanks with the same footprint but less height, specifically the 40 gallon long and the 33 gallon will give you capacities of 44.9 gallons and 35.6 gallons when using outside measurements. Taking inside measurements to measure water holding capacity will tell you that the 40 gallon long has a water holding capacity of 38.3 gallons, and the 33 gallon aquarium has a water holding capacity of a little over 29.4 gallons. The 40 gallon aquarium true volume is a good 95.75% of advertised capacity and the 33 gallon aquarium comes in at 89% of advertised capacity so essentially the same percent reduction as with the 55 gallon aquarium.

One tank in particular in the Aqueon line has a very very good capacity in comparison with it’s advertised capacity; that is the 40 gallon Breeder. Outside volume measurement yields a volume of a whooping 48.7 gallons. Inside measurement yields a capacity of just about 42.2 gallons so even with the internal volume reductions the true capacity is better than 5% more than advertised capacity.

Other Aqueon tanks that have true volumes greater or very close to advertised include the 10 gallon, 15 gallon, 20 High and 30 gallon long aquariums. Others miss the mark by a bit more, but not to the extent of the 55 gallon or the 33 gallon aquariums.

Barry Simon


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  • aoiumi
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Excellent information! Important for everyone.
Wow this is so cool!
Amazing reasoning and is just amazing to read.
fantastic information!
i am very impressed.
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