# Population level "Rule of Thumb"



## Manwithnofish (Mar 12, 2008)

Since we have nothing to go on other than a "Rule of Thumb" regarding the number fish a given tank can support. And since we tend to push the limit by buying fish smaller than they will grow, and the fact that we would like to have as many fish as we can. What are the telling signs when we start approaching that limit. How do we know that it's time to reduce their numbers. What are the limiting factors from the fishes point of view? Can we identify the limiting quantity(s) and increase the carrying capacity by increasing the limiting quantity(s)? I know this is a multivariable issue with no simple answer, but how did anyone derive at the 1 inch / gallon rule?

This is a generic question, so please don't ask how many fish I have or how many gallons my tank is, or how large or what species, or anything specific about my tanks.


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## Tex Gal (Nov 1, 2007)

It seems to me that there are really 2 main issues when it come to stocking our tanks.

1. Water quality. There has to be a way to handle the fish wastes. With good filtration and water movement we have increased the amount of fish we can put into our tanks. Not to be forgotten is the wastes that fall to the substrate. How do we handle that - snails, vacuum cleaning, lots of plants.

2. Territory for the fish. Each fish needs a certain amount of room for it's own territory. Some fish will kill others protecting their territory. Others will get stressed because of overcrowding and succumb to illness and die.

It's relatively easy to control things like enough available food, water movement, Oxygen supply etc. Water quality is not too hard if we are willing to have plenty of plants, do w/c, clear out debris and test water parameters often. I think the fish territory is the most difficult. If we have too many fish for the comfortable living of each fish they will get diseases, fight each other, etc and will end up achieving the right mass by dying. That's never fun. One way or another you'll learn your own tank and fish. Then you can just have fun!


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## ranchwest (Jul 17, 2005)

Tex Gal gave a very good response.

I have a response that may be somewhat simplistic, but when I have so many fish that it is not aesthetically pleasing then I have too many fish. This would include having too many varieties of "swimming" fish.

Up to that point, I just have to do the things that Tex Gal said to maintain the tank.


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## Jookie (Sep 30, 2003)

Plus more plants = more fish. The other thing you will notice is that with more plants, it looks like you have less fish because they are darting in and out, hiding, whatever. So I think that if you have a lot of plants it looks okay to have more fish, where a bare tank would look silly with an equal number of fish.


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## Heady (Mar 4, 2003)

I've noticed that in lightly planted tanks that fish start to die off if I deviate too much from the 1 inch per gallon rule. It was probably just from fishkeepers' experiences.


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## Six (May 29, 2006)

I agree. When there are health issues, it's time to dwindle the numbers. Re-occuring health issues is a sure sign of overstocking for the maintenance and tank size.


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## JanS (Apr 14, 2004)

I don't know who came up with the inch/gallon rule, but it's a terrible rule that I wish would just go away.

The difference in fish volume between one 8" Silver dollar and eight 1" tetras is staggeringly different, so I shudder when I see someone buy a large bodied fish and think that rule can possibly work.

The other thing that makes me shudder is when someone buys a juvie fish when it's tiny in the store and think that's when they can count body size. As you stated, you have to look at the full adult size when figuring stocking levels.

In addition to the good advice you've received above, my general rule of thumb is the smaller the tank size, the smaller the fish I put in there. I consider even a 30 gallon tank small, so the biggest fish I have in my 30's are only about 2" long at maturity, and most of them are smaller (like Cardinals).

Small fish = lots less waste.


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## ranchwest (Jul 17, 2005)

JanS, you are so right. Rigid rules are misleading.

My general guideline for beginners is similar to yours -- small tanks are suitable for small fish. A simple guideline might be that the maximum fish size for an individual fish is one inch of fish per one foot of tank. Even then, such a guideline should not be regarded as a rule. A fast, active, strong-bodied fish needs more space than a slow, sedentary, modest-bodied fish -- and that's just swimming requirements. There's also waste management requirements.

One exception to most rules that comes to mind is an angel. To judge only gallons or feet is incorrect because angels need vertical space possibly even more than horizontal space. A twelve foot long tank that is hundreds of gallons is not enough if it is only a foot tall.


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## BryceM (Nov 6, 2005)

The actual quantity of ammonia and other organic wasts produced by a fish is a function of their mass and their metabolism.

Fish mass and fish length have very little to do with each other. Compare a 3" loach to a 3" oscar. It would probably take 100 cardinals to weigh as much as one mature altum angel.

Metabolism is a bit harder to quantify, but in general fast, speedy fish will burn through their calories faster than fat, slow fish.

Once you keep planted tanks for a while it's pretty easy to see when something is too crowded. The first clue is recurring algae problems. The second is that you can't see the plant's for all the fish in the way.

It's almost always advantageous to limit the number of species too. I have 9 species in my 180g tank and I would be very hesitant to add any more.


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## Danielle (Jan 26, 2008)

So what about some of the heavily populated nano tanks I've seen here then? For example... a 3 gallon cube with 10 small tetras (.8"), or a 4 gallon with 18-20 CPDs and a small population of shrimps?


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## ranchwest (Jul 17, 2005)

The key is to manage the tank. Beginners should keep the fish population and feeding low. With more experience, you find out what the limits are and WHY.

Light, fish/invertebrate load, feeding, plants, water quality, fertilizers and plant treatments, filtration, it all fits into one picture that needs to be managed.

Some of us like to push the limits. lol


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## JanS (Apr 14, 2004)

Danielle said:


> So what about some of the heavily populated nano tanks I've seen here then? For example... a 3 gallon cube with 10 small tetras (.8"), or a 4 gallon with 18-20 CPDs and a small population of shrimps?


Some people really tend to over do it with nanos too, so as stated a light population to match the tank is always best.

I have a 12 nano reef (2 or 3 years old), and it has 1 small Percula clownfish in it, and that's all I'll ever add. Could I get by with adding another small fish or two? Probably, but I like to keep it to a minimum in case something happens like a power outage or something that would keep me from doing my frequent maintenance that keeps the tank pristine.

There are always tons of "ifs" that should be added to any scenario as a safety net.


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