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How much space does a fish need?


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#1 Flatfish

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Posted 23 July 2015 - 02:41 PM

I realize that's not a very specific question but I have been pondering it with respect to a wide range of fish sizes lately, everything from fry the size of rice (about half the size of rice, actually!) to some rather large koi, and some shiners I have.  Obviously 10 inches of fry is not the same thing as one 10 inch adult as far as biomass and bioload goes.

So how do you figure out how much space is right for a particular fish or group of fish? Surely there is more to it that just keeping your nitrogenous waste levels down?
I imagine there are also behavioral considerations? How much does amount of filtration factor in?



#2 Betta132

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Posted 23 July 2015 - 05:32 PM

What kind of fish? Some fish basically don't move unless there's food, some never stop moving. 

Spacial needs are based, first, on water quality issues. However, if you put a 3-foot-square chain link cage in a river and kept a 2-foot bass in it, the bass would have very clean water but would be extremely unhappy. 

Unless it's a fish that mostly sits on the bottom, I think a general rule is to never put a fish in a tank that's less than 5X by 2X, where X is the length of the fish. Very active fish like shiners should probably have more like a 10X by 5X tank. Inactive fish could go in more of a 3X by 2X, assuming their inactivity is constant as opposed to just them being asleep. 

Also, a fish in an aquarium should be observed for signs of stress. If they exhibit stress behavior such as unusually high aggression or nonstop pacing, they may need a larger aquarium.

A lot of the minimum tank sizes are probably based on observation, as well. People see a fish looking unhappy in X sized tank, so they move it to Y, then to Z. The fish displays no stress in tank Z, but seems unhappy in X and Y, so Z is considered more or less the minimum tank size. 

 

Basically, if you aren't sure how much space a fish needs, you go and ask people who've kept it. If nobody knows, try a tank and see if the fish looks stressed or cramped. In young fish, keep a close eye out for any signs of stunting.

 

Filtration isn't as much of a factor as you'd think. Biological filtration is a major component, as that's what handles ammonia, but mechanical filtration (the filters themselves) don't factor in too much. Mechanical filters have three purposes: They hold/oxygenate media so that BB (beneficial bacteria) can grow, they provide circulation/oxygenation, and they help remove physical dirt from the water before it becomes ammonia. Heavy filtration helps when you keep large fish, but it's not required, and it doesn't automatically make a tank able to handle big stuff.



#3 Matt DeLaVega

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Posted 23 July 2015 - 08:21 PM

It is very subjective as mentioned, and also personal preference.  No real hard rules, most use common sense and a degree of trial and error. Start with a lightly stocked tank and move up from there. Filtration needs go up with biomass.

 

Aggression, activity levels, size all play a part. You may only be able to keep a single sunfish in a 55. You might be able to keep several big river chubs in the same space.

 

You are asking the right questions, just not sure there are really any right answers.


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#4 Josh Blaylock

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Posted 24 July 2015 - 10:24 AM

Matt is right.  There is no hard and fast rule here.  There are so so many factors that contribute to how much space a fish needs, or how many fish you can keep in a certain sized tank.  Basically everything in an aquarium is a factor, from the bottom to the top.  Best thing to do is research what other's have experienced and try things yourself.


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#5 Flatfish

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Posted 25 July 2015 - 04:28 PM

Thanks, Betta132, that's the most answer I've been able to get so far, though much of it is what I already suspected was the case but wanted to hear from more experienced keepers.  The type of fish in question include goldfish fry, koi fry, very large koi, cichlids, and shiners. 

 

And thanks, Matt, glad to know it's not a stupid question! 

 

I will point out that when I say filtration I'm referring to biological filtration, as mechanical is basically to (a) support the biological (and the oxygen thing, but not sure if that applies to canisters?) and (b) aesthetics; it seems like it would only help with water quality if it is mechanically filtering stuff that is going to break down into ammonia AND you are removing it from the filter regularly. 

 

Asking people about specific fish has very often let to more confusion than clarity due to the variation of answers I've gotten, often with little to no explanation to them.

 

You said keep a close eye on any signs of stunting; if you don't know how fast a fish SHOULD be growing, is there another way to judge that?
 

Even more complicated, I find, is figuring out how much space for inverts, since their impact on bioload is relatively lower than for fish.


Edited by Flatfish, 25 July 2015 - 04:29 PM.


#6 Matt DeLaVega

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Posted 25 July 2015 - 05:10 PM

 I have kept a pound of fish per ten gallons of water in a greenhouse aquaculture system with no trouble, and believe I could have pushed it much farther. Have pushed it even more indoors in my 240. I honestly cannot remember for sure but think I had roughly 30 pounds of fish in it when I shut it down and weighed them. So that gives you an idea of what an amateur aquaculturist can do. I think the pros go a fair bit higher. Surely you do not want that kind of density in an aquarium, but it gives you an idea of what can be done. So go as far as you feel comfortable.


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#7 Flatfish

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Posted 27 July 2015 - 11:37 AM

Wow.  Did you do that by increasing filtration?  Were they schooling fish?



#8 Matt DeLaVega

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Posted 27 July 2015 - 04:43 PM

Tilapia. Yes a huge amount of bio and mechanical filtration. A 500 gallon tank packed full of greenhouse shade cloth and roughly a 75 gallon fluidized bed filter, more or less then 100 or so sq ft of plant growing area. Pretty extreme, but I am just using that as an example..


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