# 20g per FISH RULE



## rbP NUT (Dec 2, 2003)

i have heard as im sure many others have the its 20 gallons of water per piranha, i know the tanks foot print is of importance to, but i was thinking how did this come about, does the fact that 20g of water holds enough bacteria to cope with the fishs waste, because over crowed tanks often have more water problems such as ammonia & nitrIte spikes? other than volume and space for the p to live comfortably does the 20g a p hold any deeper meaning?


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## Chunkis (Nov 2, 2003)

I think its for territory reasons, (if ur talking about piranhas). you wouldnt want to put a RBP or any piranha in a 20g. its too small. Mybe if somone had a 100g tank...tehy could have 5 piranhas.20g per piranha of that 100g tank would be for swimming and exersize, and just room for staying out of other fishes faces.

Also, mybe somone was smart and didnt want newbs to think they could put a full grown Red Devil in a 10g and have it live. So they made up the rule "20g per fish"


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## Death in #'s (Apr 29, 2003)

:laugh: its just a assumption the rule can be bent to a piont
there is no rule of thumb just an assumption and 20 is a nice round number instead of 18.8g per fish


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## rbP NUT (Dec 2, 2003)

Death in # said:


> its just a assumption the rule can be bent to a piont
> there is no rule of thumb just an assumption and 20 is a nice round number instead of 18.8g per fish


 so the number got picked out the hat?lol
chunkis thats a good point


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## Judazzz (Jan 13, 2003)

I think the 20g per fish rule is just a guide line (and only applies to pygo's - no serra will be comfortable in a 20g for life) - but since tank gallonage doesn't say anything about the tank dimensions, it's kind of sketchy (a 90g high will not support as much piranha's [all fish for that matter: the smaller the surface area of a tank, the less fish it can sustain] as a 90 long tank...)

Personally, I prefer to use tank footprint as guideline:
- for a solitary fish for life, about 4-5x the fish max. size in tank length and 1,5-2x that size in depth (with a minimum of 16", and with a maximum of 24", since deeper tanks are not that easy to come by). For small to medium-sized fish, a height of 16-20" is fine, for larger fish 20" or more...
- for pygo's, the minimum length should be 40" (for 3-4), and tank depth of 20". For each extra fish, I'd add about 8" in tank length. With 6 or more, a depth of 24" is fine.

All this is just my opinion... :smile:

*_Moved to Equipment Questions_*


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## JeFFLo (Aug 29, 2003)

i think the 20 gallon per fish rule should only apply to serras if you are gonna try to shoal them. pygo's are schooling fish so i dont think they each need 20 gallons of space since they will be in schools anyway. this doesn't mean that it is ok to put 5 reds in a 20 gallon tank. but say a 55 gallon tank...if you look at it, it is definitely big enough to house more than 3 pygos.


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## rbP NUT (Dec 2, 2003)

intersting views guys, i thought serras would need major space in an atempt to shoal? alsodoes any one know if there is a maximum amount of bacteria that 20g can hold and how much bacteria is needed to handle one fishes by products?


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## Scooby (Dec 25, 2003)

Hmmm yeah i have heard also no smaller then 40Gal tank for P's... But my buddy has a 20 gal and at first i didn't think it was a prob but now when i look at his fish probly have enough room to do a circle and thats about it, since all of his deco's and all.... i keep telling him to sell his small tank and purchase a bigger 1 bust he is one stubborn ass mofo


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## JeFFLo (Aug 29, 2003)

it matters on the tank too not just the gallons...some tanks are long, some are tall, and some are wide.


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## rbP NUT (Dec 2, 2003)

no i do realise tank foot prints are more important in choosing a tank then volume, but its the 20g thing that has me stumped, i really thiught there would be some scientific explanation to it all.


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## Judazzz (Jan 13, 2003)

rbP NUT said:


> no i do realise tank foot prints are more important in choosing a tank then volume, but its the 20g thing that has me stumped, i really thiught there would be some scientific explanation to it all.


 I think someone just came up with it one day (probably a knowledgable someone who's respected within the hobby), and after that it started leading its own life.
It sounds pretty decent, imo, since it's not an outrageous thing to use as a guide line - it's just that there's more to it than just gallonage.


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## rbP NUT (Dec 2, 2003)

fully aware on the other factors in regard to housing a p, but i thought that 20g was a given measurement of water for sufficient bacteria to thrive and space factor regardless of the dimension. thats all, thanks guys


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## JeFFLo (Aug 29, 2003)

do you mean the bio load?


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## rbP NUT (Dec 2, 2003)

JeFFLo said:


> do you mean the bio load?


 thats the 1


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## BanditBrother (Nov 8, 2003)

Sound gd to me!!! Ive just got rid of my two large plecs to give more space to my fish!! They seem alot happier wiv more space!! The 20G rule is a gd estimation 2stick2!! Take note guys!! Before u buy!!


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## jackburton (Nov 25, 2003)

i agree but its not carved in tablets of stone


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## Chunkis (Nov 2, 2003)

I think....33g minimum for 1 RBP, 40 for 2(u would never do that htough) and 20g after that for another RBP.


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## alphozo (Dec 12, 2003)

so in larger tanks could you stretch out the 20g rule? how many pygos do you think that you could fit in a 125 say about 5-8 inches a piece?


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## Chunkis (Nov 2, 2003)

7 or 8...mybe 9 if u were lucky


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## Judazzz (Jan 13, 2003)

alphozo said:


> so in larger tanks could you stretch out the 20g rule? how many pygos do you think that you could fit in a 125 say about 5-8 inches a piece?


 Depends on the footprint: if it's a 16" deep and 26" high tank (hypothetically speaking), not nearly as much as when it's a 24" deep and 20" high tank.


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## sweet lu (Oct 3, 2003)

i hate that 1 gal per 1 ince of fish rule. the 20g a rule is much better.


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## Innes (Jan 13, 2003)

rbP NUT said:


> i have heard as im sure many others have the its 20 gallons of water per piranha, i know the tanks foot print is of importance to, but i was thinking how did this come about, does the fact that 20g of water holds enough bacteria to cope with the fishs waste, because over crowed tanks often have more water problems such as ammonia & nitrIte spikes? other than volume and space for the p to live comfortably does the 20g a p hold any deeper meaning?


 gallons per fish rule have no real baring to fishkeeping, it is possible to have 20g of water and it not even be suitable for a neon tetra....

the improtant things are space and surface area.


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## airtorey15 (Jun 15, 2003)

it sounds like a well thought up rule and has many good points ass we see when we talk about it here.


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## Wulfgar (Jan 2, 2004)

Judazzz said:


> alphozo said:
> 
> 
> > so in larger tanks could you stretch out the 20g rule? how many pygos do you think that you could fit in a 125 say about 5-8 inches a piece?
> ...


 TRue. I had 20 adult rbp in a 200 g. & they got along just fine. 4 or 5 in a 58 is was also good 2 go.

WG


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## upt1me (Jun 26, 2003)

I created the 20 gallon rule and propigated it out around the internet. Now it has become the rule of thumb.


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## vlahos (Sep 7, 2003)

i personally like giving more space than just 20g per fish. again footprint is most important. getting a 90g is in my opinion not more beneficial than getting 75g because it has the same footprint but is just taller. so just because you have another 15g to play with doesn't mean you could put in another fish.
personally, for serras, a footprint of 48"x18" is good for life of most species
as for pygos if you're going to shoal i would think 60"x 20-24" is best


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## ralphinnj (Mar 2, 2003)

One last thing to integrate into your tank size requirements: filtration, water changes, etc. If your water is perfect all the time (means great filtration, frequent water changes, etc. etc.), you'll be able to support more fish. I think what most of the experienced people here are talking about assumes good water conditions. Another factor is food supply. Putting aside the mixing of incompatible species, you still need to provide ample food, ample variety of food, etc. That is, you can have 4 rbp's in a 200g tank and they will kill each other if you don't feed them. Vice versa, you probably could get them to survive in a 55 (or even smaller) if everything was perfect. If you think of them as you would a dog or a cat, you won't overload the tank because it is still an unpleasant existance (would be like keeping your cat or dog in a closet all of the time and keeping it clean and giving them good food; theoretically, they will survive just fine, it just won't be much fun).

RL


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## andymel (Oct 27, 2003)

rbP NUT said:


> intersting views guys, i thought serras would need major space in an atempt to shoal? alsodoes any one know if there is a maximum amount of bacteria that 20g can hold and how much bacteria is needed to handle one fishes by products?


 Bacteria and biological filtration is not dependent on tank size. It depends more on the type of filtration (surface area) you are using and the amount of water moving through it. The 20 gal rule should only be a guideline as some p's will require more space than others. You need to find out the max size of the fish and the type of behavior in the wild and use that to estimate the tank size. If you are concerned about water chemistry as opposed to space requirements of the fish then all of these things play an important role:
filtration(wet/dry filters can add significant water volume while the tank size doesn't change)
types of food and frequency of feeding
plants


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## nitrofish (Jan 14, 2003)

rbP NUT said:


> intersting views guys, i thought serras would need major space in an atempt to shoal? alsodoes any one know if there is a maximum amount of bacteria that 20g can hold and how much bacteria is needed to handle one fishes by products?


 thats a hard one because a tank will only have as much bacteria as it needs to break down wastes. so a 100 gallon with a single 3" piranha has as much living bacteria as a 20 gallon with that same fish


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