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Old 01-23-2022, 07:21 PM   #1
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“Calculate” Fish Capacity (vs Total Tank - Earth/Air/Fish Tank Cap.

I am pondering how substrate and fill levels (Earth & Air) will displace water as I plan a custom tank order.

I want a solid idea of what I really have to work with, so fish:water ratio is optimum. I like math, and Excel, so I created an EZ-fill worksheet (happy to share if you like).

I would love to hear other things I can measure before ordering a tank. (And a good custom builder.

The sheet pictured calculates a tank's usable capacity in gals. (I call it Life-Supporting Cap.*) vs. its actual total capacity, based on:

• its actual interior (my direct L, W, H measures), and
• inches of substrate and fill-level-to-brim

I enter these into the two green cells, the only ones I need to touch once int. L/W:H is noted.

L and W of the interior floor (area), multiplied by total displacement H in inches (earth + air), produces the volume to subtract from your total tank capacity.

Another version allows me to set a Golden Ratio of L to W, and input my Fish Capacity, to determine both the adjusted volume and total actual volume, AND the interior dimensions, and use glass thickness to calculate the exterior.

Questions are welcomed.


* Maybe “Fish Capacity” is better, since gallons of water and inches of fish are 1 to 1, and that is how some people determine the gallons of tanks they buy.
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Old 01-24-2022, 03:57 AM   #2
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1 inch per gallon is a very rough and ready measure of calculating aquarium capacity. It can be useful for estimating such things, but should be taken with a pinch of salt and shouldn't replace actual experience of keeping individual fish.

Its a good gauge for judging if an amount of water of will support the bioload but doesnt account for other needs of fish.

For example, you couldnt keep a 10 inch fish in a 10 gallon tank. But you may be able to keep 10 x 1 inch fish in a 10g tank or 10 x 10 inch fish in a 100 gallon tank. A fish needs a certain amount of space to comfortably live in, have enough room to swim around in. Too small a tank will stunt fishes growth, lead to ill health and shorter lives.

Some fish produce more waste than others and the 1 inch per gallon wont work so well there either. Temperate fish are messier than tropical fish so you cant stock as heavy there. Some fish need better water conditions than others, so again you shouldn't stock as heavily if you plan on keeping more sensitive fish species. So 1 inch per 2 gallons or 1 inch per 5 gallons may be a better ratio.

Most of the small tropical fish people keep are social fish. They do better if kept in groups. So while a small tank might be able to support 1 or 2 small tetras from a bioload PoV those fish need to be kept in groups, so a bigger tank is needed to support the group. These small fish are also active swimmers, so we go back to the comment regarding a fishes other needs and they need a big enough tank to support their active natures. You may need a 20g tank for an active tetra, that tank may support 20 of those tetras, or 3 groups of 6/7 tetras and thats where community tanks arise from.

Take a look at AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor

While this again isnt a fool proof method of calculating aquarium fish capacity, it does take account other factors than bioload such as filtration, space needed, it gives some advice on numbers of fish needed and compatibility between fish species. The database has been built up over a long period and many experienced fish keepers have had input into it and continue to do so.
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