Why Do Some Fish Require A Minimum Size?

Hi, This might seem a stupid question but I'm an absolute beginner and want to understand as much as I can. Why do some fish require a minimum size? Obviously larger fish speak for themselves. But smaller ones for example figure 8 puffers are recommended minimum size of 15-20g, yet the fish is tiny. Why would it need a larger tank? and what would happen if it was kept in a smaller tank.

 

It needs a larger tank so it can swim around. For example, Cory cats are tiny but they need at least a 20 gallon because they need room to swim around. Look at it like this, would a hyperactive toddler want to just have a closet to run around in or a field.

 

Hi, This might seem a stupid question but I'm an absolute beginner and want to understand as much as I can. Why do some fish require a minimum size? Obviously larger fish speak for themselves. But smaller ones for example figure 8 puffers are recommended minimum size of 15-20g, yet the fish is tiny. Why would it need a larger tank? and what would happen if it was kept in a smaller tank.

All fish have a minimum tank size, but no maximum.

 

Some fish are also irreversibly stunted in their growth by too small of a tank. A good example are goldfish their internal organs are squeezed into stunted bodies, the body won't grow but the organs keep growing causing premature death. (in nature can live 15+ years)

 

Hi, This might seem a stupid question but I'm an absolute beginner and want to understand as much as I can. Why do some fish require a minimum size? Obviously larger fish speak for themselves. But smaller ones for example figure 8 puffers are recommended minimum size of 15-20g, yet the fish is tiny. Why would it need a larger tank? and what would happen if it was kept in a smaller tank.

being a cichlid owner I can give you an example,
say for example my Midas cichlid, who will reach 15"(hopefully), and many things go into deciding tank size. He's in a 150g. One thing is footprint of a tank, say you have a 55g, and a potentially 15" fish, like king Midas, the depth of that tank is only 12". So he wouldn't have any swimming room. So room to swim is another big thing. Another is bioload. A 55 gallon may be a lot more work to maintain because of less space. And toxins and all from organic waste breaking down may effect a 55 gallon a lot faster than say a 100g. Another thing is for aggresive fish, territory is often an issue, so if you want tank mates, you may need much more space to accommodate all your fish, to be sure they don't fight over space and territory. Even something like a three spot gourami, needs a good size tank because they are aggressive towards other sometimes. Another factor is the fish itself, it you put a big fish in too small a tank,it'll be sure to make short work of that tank. There's just a lot of things to it. Everything from footprint, to bioload, to territory etc.another is like the above said, some fish get stunted by too small a tank.

 

Pretty much covered in previous replies, but to simplify- it depends on how active the fish is, how big a territory (a territorial fish) needs, and how much waste they produce. Goldfish are an ideal example. You hear all the time, "but mine's small, can't I keep it in a five gallon tank?" Still, it's a baby version of a giant fish so consumes ridiculous amounts of food to grow quickly, and therefore produces more waste than a fully grown 'small fish.'

 

Huh. Watching this forum as I've gathered a lot of information. I always assumed that it was because many of those smaller fishes are more sensitive to changes in parameters, therefore need a large tank that could keep it stable--especially if you're a beginner who doesn't know enough to.

I keep a scarlet badis in a very mature and heavily planted 2.7. He's been doing great so far (3 months and knock on wood)

 

Another thing no one has mentioned is so schooling fish have enough room and don't become aggressive towards each other.

 

Truth is, it's all a matter of opinion.

 

Truth is, it's all a matter of opinion.

This is actually misleading as a statement. It is far from the truth. I don't mean to be rude, but facts, learning and experience are real things. You have to ground information in facts.

You can take two 2 inch fish. One will need a large tank, one a small one.
Odds are, the small tank fish will have evolved in swamp conditions, with still, shallow water, low oxygen and tiny food animals. Based on knowledge of the habitat, you can determine it will do fine in a smaller tank. The Badis mentioned above is a case in point.
If you don't want to look to nature, hobbyists have kept most available species for decades, and recorded their observations. They are very easy to find.
To keep a group, you may need a larger tank, as well documented aggression and territoriality kick in and add to the planning.
Nothing stops you from adding those fish to a large tank.
Sensitivity to environment also kicks in. My Congochromis sabinae are small and not big swimmers, but if I were to put a fish that sensitive in a tiny tank, it would crash. Collection data from its habitat says that (extreme low pH, almost no minerals in the water). I need a 20 for mine, and I was watching them court this afternoon.
Take a 2 inch African Procatopus lampeye - a fish that has silver/blue camouflage, saying directly it comes from sun dappled water with ripples. Its camouflage offers info, and so do African collectors. It's there to look up - not an opinion. You can find multiple sources saying the same thing, and offering proof as to why.
They thrive in longer tanks with flow, and die quickly in low oxygen. That is not an opinion, it is backed by natural history and 70 plus years of aquarists keeping them and recording results.
Rummy nose tetras are similar. They like space, and will die in a 2 gallon. My proof? Watching film of them in the wild, reading collection reports, talking to fishers, reading a large volume of hobbyist articles.

There is science behind a lot of our choices. All we need is the energy to look. It's not collected in one place for convenience, although older aquarium books like the Baensch Atlas series made a pretty good try. They identified opinions as opinions if the fish had not yet been kept, but based most of their suggestions on decades of research by skilled aquarists, as well as native fishers and scientists.

Opinions are when I say killies are prettier than tetras. Facts are when I say Fish A is a rapid swimmer, and people have learned it needs moving water and room not to bash into the glass.

 

This is actually misleading as a statement. It is far from the truth. I don't mean to be rude, but facts, learning and experience are real things. You have to ground information in facts.

You can take two 2 inch fish. One will need a large tank, one a small one.
Odds are, the small tank fish will have evolved in swamp conditions, with still, shallow water, low oxygen and tiny food animals. Based on knowledge of the habitat, you can determine it will do fine in a smaller tank. The Badis mentioned above is a case in point.
If you don't want to look to nature, hobbyists have kept most available species for decades, and recorded their observations. They are very easy to find.
To keep a group, you may need a larger tank, as well documented aggression and territoriality kick in and add to the planning.
Nothing stops you from adding those fish to a large tank.
Sensitivity to environment also kicks in. My Congochromis sabinae are small and not big swimmers, but if I were to put a fish that sensitive in a tiny tank, it would crash. Collection data from its habitat says that (extreme low pH, almost no minerals in the water). I need a 20 for mine, and I was watching them court this afternoon.
Take a 2 inch African Procatopus lampeye - a fish that has silver/blue camouflage, saying directly it comes from sun dappled water with ripples. Its camouflage offers info, and so do African collectors. It's there to look up - not an opinion. You can find multiple sources saying the same thing, and offering proof as to why.
They thrive in longer tanks with flow, and die quickly in low oxygen. That is not an opinion, it is backed by natural history and 70 plus years of aquarists keeping them and recording results.
Rummy nose tetras are similar. They like space, and will die in a 2 gallon. My proof? Watching film of them in the wild, reading collection reports, talking to fishers, reading a large volume of hobbyist articles.

There is science behind a lot of our choices. All we need is the energy to look. It's not collected in one place for convenience, although older aquarium books like the Baensch Atlas series made a pretty good try. They identified opinions as opinions if the fish had not yet been kept, but based most of their suggestions on decades of research by skilled aquarists, as well as native fishers and scientists.

Opinions are when I say killies are prettier than tetras. Facts are when I say Fish A is a rapid swimmer, and people have learned it needs moving water and room not to bash into the glass.

No not at all. Admittedly, my opinion was short. I don't take any disagreements here personally. Very well said.

 

There are a lot of reasons really. I personally try to stick with activity level, bioload, and size of fish. For example Neon tetras are small but really active little guys which is why I feel they need at least a twenty gallon to swim around in. Mollys are also small, but have larger bioload so 29 gallons or larger is needed to maintain stable parameters with them. Of course it gets even more complex when one is speaking of community tanks then you have to think about having enough territory space for the different types of fish, as well as things like temp. compatibility.

 

First of all very well stated by Navigator. Scarface's statement as written is misleading. But I do agree that opinion and personal preference enters when you actually try to integrate our "rules" into a stocking plan. I'll provide an example.

You will read here most every day that cory cats's should be kept in a 20G minimum tank, with a sand subtrate and in a group of 10 or more. Fair enough but in a 20G that is half your bioload used up because you want corys. I happen to have a 60G community, fine gravel substrate, planted and 1/3 of the tank floor aquascaped with some large rocks. I only have 5 cory and they are the most active cory I have ever experienced. They spend almost half the time playing in the rocks, and their barbells look perfect to me.

Anyway, I believe it best to soak up the knowledge of experienced fish keepers before you. I don't deny the rules, I just apply common sense and my own experience to them. I'm never afraid to try new things that don't line up exactly with what I read on the internet..

 

Truth is, it's all a matter of opinion.

Good to know. I've got a one gallon goldfish bowl and I'm going to bend a 12 inch goldfish to fit inside it. Since it'll already be bent it can swim in circles. It'll be fine with a weekly water change .

 

Good to know. I've got a one gallon goldfish bowl and I'm going to bend a 12 inch goldfish to fit inside it. Since it'll already be bent it can swim in circles. It'll be fine with a weekly water change .

We are past that post and nobody is bleeding California.

 

The OP has learnt a lot of lessons on his first post, a priceless first post lol I enjoyed this. The most valuable thing learned here is that you get right and wrong info, not even just here but everywhere, the best thing anyone can tell you when starting out is do your own research, ask your questions then cross check all your own research and the answers you get. Don't get fooled, be sure on stuff before making purchases or commitments.

 

The OP has learnt a lot of lessons on his first post, a priceless first post lol I enjoyed this. The most valuable thing learned here is that you get right and wrong info, not even just here but everywhere, the best thing anyone can tell you when starting out is do your own research, ask your questions then cross check all your own research and the answers you get. Don't get fooled, be sure on stuff before making purchases or commitments.

Very well said, and I might add there are some gray areas. But those are for experienced fishkeepers willing to take a risk after doing plenty of research and with a good backup plan because they don't always work. A good example being betta sororitys. But some members will talk like they're always ok.

 

you need to take into consideration not only their size but also their activity level and bioload. higher the bioload the bigger the tank, higher the activity level the bigger the tank

 

I'm going to go full nerd here...
Everyone duck.
I think, following OnTheFly's great post above, that we have a rules problem in our hobby. 5 Corys are fine to me. 6 is an arbitrary, made up number. Why not 7?
If a fish in nature has a temperature range from 20-27c, then that is its perfect temperature. There is a sliding scale - maybe it breeds seasonally at 26, so if you want to breed it, 26 is your magic number. 21 is good for maintenance. So is 27. But an ideal? There's no such thing - just a sliding scale based on facts.

We love rules, and I think that's what scarface is shooting at.

Tank size isn't a good example, because it is researchable and works case by case, species by species, but some of the other rules are not fact, and have no research or practice behind the rule. There is no sensible reason why '6' Corys. What there is is that we know Corys are social and prefer groups. Often in nature, those groups are enormous. A friend of mine stood in a Brazilian creek during a Cory migration, and said they passed for as long as he could stand there. He said there were many thousands. That would look good in my tank! But after that, 4 seems small. That statement is an opinion.
Somehow, the once reasonable statement "Corydoras are social and prefer groups. Six seems a starting point to work with" has been transformed into a dogmatic rule of six.
Now, if you want to breed a hard to sex or young species, six is the minimum number to buy to have a good chance of randomly getting two sexes. That's math, and is probably the root of the rule of six. A breeder says you need six and it becomes a rule in a different context.
That's the human mind when it wants answers.

The first time I got to see platys and mollies in the wild was on a vacation in Belize. The driver was going like his hair was on fire, and noticed me glancing at the speed limit sign. He grinned and said "Here, we see the speed limit as more of a guideline than a rule." I always remember that guy when I try to answer a question here.
I also remember that he hit a speed bump, which was objectively and solidly painful.

 

I wish threads like this were required reading before you get your forum posting license. I have only been on fish forums for about a year. The strict rules thing is our biggest problem IMO. Well at the very least it annoys me when somebody states they have 5 neons and somebody advises them they must add one more. I do wish we discussed where our "rules" come from. I am almost sure you are right about where the rule of six came from. The cory debate annoyed me enough to cause me to move fish around my many tanks and leave them there a month. I needed to validate (in my mind) what was probably true. I put single corys in tanks from 5G to 60G. They predictably lay around like a slug most of the time. Make it three and they still don't do much in a small tank. Put the same three fish in a 60G and they get far more active. Add a couple more and they are now very happy and fly around the tank foraging almost continuously. We'll never duplicate 5000 corys in a massive school in acres of water. I don't know why we pretend we can. There is no practical difference between 5 or 10 cory in an aquarium, but I still read it here VERY often like it's validated fact. It's simply not.

I have the similar opinion on guppies. Schoolers? No, of course not. But there is sure a big difference in how they act if there are a half dozen or more, rather than one or two. If you only have a couple they surely won't fall ill though. It seems most species prefer a few of their own, but I would never suggest someone must have more of them unless they are truly schoolers.

I'm rambling so I'll stop now..

 

Not much to add that's not already been posted. But, since the OP pointed it out as an example. I used to own a figure 8 puffer, kept him alone in a 29G and he used every single inch of that tank.

Sure, he was a small fish and I could have put him in a 10G, but he would have gotten bored, they are very intelligent, the tank would have quickly been a mess without a WC at least every other day (messy eaters, especially with snails) but in the 29G it was clear he was happy.

I miss that fish.