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Old 08-27-2016, 01:55 PM   #1
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How big of a fish can go in my tank?

I can not begin to say how many times this question comes up, but every time it does, this is a fairly serious question that needs to be addressed. so to try and address it, I've 'made up' a system that I've been using since no matter how hard I look, I can't seem to find a solid answer. I wanted to run it by you guys here, and if anybody from saltwater wants to chime in that would be fine too, but for now I am more specifically targeting freshwater.

this is not going to cover the number of fish you can keep. merely the 'max size'

I started off with a "max sized fish for your tank rule" I found somewhere awhile ago that said the tank should be at least 4 times as long as the max size of the fish and 2 times as wide as the max size of the fish. this is good because it accommodates the fish and allows it to be able to move fairly freely even with most typical decorations in the tank. however this is not suitable for more active fish. yet, for most "lazy" catfish and plecos that spend the majority of their time stationary, this may not be the worst guide. so for moderately active fish that would passively search around the tank, such as many south American cichlids, I would divide the smallest of the previous numbers by 2, and to keep highly active fish such as rainbow fish, divide by 2 a second time.

to help clairify this with actual numbers , let's look at a standard 55 gallon tank. this is a fairly common size for people to begin wanting larger fish. they are typically 48" long and 12" wide. now 48/4 = 12" and 12/2 = 6". so idealy the largest size for "lazy fish" would be around 6". and this divided by 2 would mean the largest moderately active fish that should be in there should be around the 3" mark. and for rainbows or other highly active fish this would mean 1.5" is about the largest they should be in a 55.*
I will now do the math for a few other standard tanks and put that below so that you can look at the numbers and see how you all feel about them, however, what I really want is feedback on how this could be modified to help provide more ideal yet realistic conditions to all fish.

approximate maxes for activity levels
*tank size *Lazy *Moderate *active
10 G -5 -2.5 -1.25
20 High -6 -3 -1.5
20 Long -6.5 -3.25 -1.625
29 G -6 -3 -1.5
30 G -6.5 -3.25 -1.625
40 breeder -9 -4.5 -2.25
55 G -6 -3 -1.5
75 G -9 -4.5 -2.25
125 (6ft) -9 -4.5 -2.25
180 (6ft) -12 -6 -3

(sorry about the long post and the formatting of the numbers. I didn't know how to do it any better
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Old 08-29-2016, 02:21 AM   #2
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10g are tricky. You can't keep a 5" fish in a 10g. Too much waste.

I advise either a Betta
Or true Nano fish near 1" as adults
Or Shrimp

3 Platies might survive in a 10g but they may die young.

JMHO


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Old 08-29-2016, 04:09 AM   #3
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A really good point about waste.

You need to figure in the waste of a fish like a same length 4-5" common Pleco or a Goldfish would be more than a Siamese Algae Eater. They are all fairly active fish.
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Old 08-29-2016, 03:10 PM   #4
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awesome! I love the responses! but keep in mind I can't say that I would want to put anything 5 inches long in a 10 gallon. and I'm not really accounting for waste. at this point I'm looking for a rule that you can look at and say that in a tank of X long and Y wide, a fish of 'Z' length will feel comfortable in it. this is merely a proposal of sorts to try and get some gears turning on a real generalized rule for the size of fish that should be allowed in a fish tank with set base dimensions which is all that matters (at least to me in most cases) since few fish genuinely need height of a tank (as long as it's around twice their own height.). how would you guys propose modifying my rule? or do you have a better one in mind already? i'd love to hear it!
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Old 08-29-2016, 05:15 PM   #5
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I think I would have some factors for things like that. I still use the inch per gallon guideline on small fish, community stocking but I'm modifying it on the fly for various factors like tank zone (eg bottom), messy or not, body shape, schooling fish, temperament, etc and where it is not sensible for say small tanks (limits for use). For example a goldfish would be technically possible for my tank but as its set for tropical (and probably more importantly for me - a small fortune spent to plant it), a goldfish is out.

One thing I did like with the old guideline was inclusion of tail fins and when full grown (re-homing being harder here). It's probably the main part I've kept to remind myself to think what the stocking will look like not just now when I get the fish but in say a years time (and not just buy all the pretty fish I see ).
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Old 08-29-2016, 08:02 PM   #6
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while yes I would suggest different fish at different stocking densities and different tank requirements just because I know how they live or what they're going to live with, I would also love to try to pin down a rough 'rule' of sorts that I could comfortably tell people to use for a 'max size' of fish. Size alone. People always want to know how big of a fish they can keep in their tank. And people would also likely prefer to be told that 'X' is acceptable and it follows a 'rule' that is widely believed to be true over just me saying 'an Oscar gets way too big for the twenty gallon tank you have running'.

But you're thinking about a lower limit on such a rule? All statements in this hobby have their limitations for where they're applicable. But where would you guys set such a limit or how would you modify the 'rule'?

(Any reply that adds to the discussion appreciated!)
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Old 09-04-2016, 02:44 AM   #7
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I agree with delapool in saying there is more that needs to be taken into account here. What I mean is that according to these calculations rainbow fish are to big for 180 gallon tanks, and African cichlids don't really fit into any of the niches (lazy, medium, small active). I do like what you are trying to do here and the thought that was put into it is really thought out so I commend you for that, but I don't think you can numerically value fish stocking due to the insane amount of variable that go into stocking a tank.


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Old 09-05-2016, 12:00 AM   #8
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I just have been dealing with this long enough to finally understand something. most of the people that you see on forums like this are NOT the kind of fishkeeper that I typically encounter. while things are complicated when it comes to aggression, habitat preferences, the individual fishs' personality, among many other really cool things! people want things to be simple. i'm not saying that my initial proposal was correct! in fact, I knew that somebody would point out how low the size for active was in a 180! (most larger rainbows are fine for a tank that size.) I just hoped to start some form of discussion toward a 'simpler' answer to a fishkeeper with a 'new cycled tank setup' who has yet to add any livestock to their tank. (I don't think size is the most important thing at all... I generally prefer unique behaviors. but this is not the case with so many people I encounter daily.)

so for all of you who disagree with my first idea, post counter ideas! or maybe just a modification! anything is appreciated. I know it's not right as it sits. this is why i'm here! to dip into the collective experience of people from across the world. to try to find a 'simple' answer for anybody starting a new tank. (thinking things will be complicated is a reason why many people never get into this hobby. i'm just trying to help .)
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Old 09-05-2016, 02:24 PM   #9
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How big of a fish can go in my tank?

I'll try to sit down tonight and tomorrow to take a look and see if I can get some proposed counter ideas/ build off what you currently have established.

Edit: I'm not saying I disagree, I'm just saying some things need to be edited a bit, I did say I like what you were trying to do here however.


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Old 09-05-2016, 02:55 PM   #10
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i think this is a good idea, but to many variables, bio waste, filtration, temperament, size, length. i mean an arowana would need a tank that is what 6 feet wide, and 12 feet long! i may be building one for an arowana that is 8 long and 3 wide, which a silver arowana would love.
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Old 09-06-2016, 01:12 AM   #11
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it is true that silver arowana really can turn on a dime for a fish their length. they would probably be fine in a tank that is not as wide as they are long because of it. however, do you want to only offer your fish a 2 dimensional space to pace back and forth in, or a 3 dimensional environment to explore and interact with?

i'm not debating the ethics of keeping fish in tanks. (even if that may seem the direction i'm going with that previous statement) i'm just trying to identify a general guide that when followed, one can be certain that a fish of a given length and activity level does have enough room, but not necessarily an 'excessive' amount of space. and if anybody can lend a hand in this endeavor, I would be most grateful!


-looking forward to what you've come up with machotaco!
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