The "inch Per Gallon" Rule

[Chad]

If you have done your research and selected a suitable species...then you have already skipped the inch per gallon rule. 

 

[daizeUK]

Okay.  But if one is inexperienced, what is the best way to determine the maximum size and number of fish their tank can comfortably support?
Our current fishless cycling guide states that "A major benefit of this fishless cycling method is that you can now fully stock your tank in one go".  So how do we define fully stocked to a beginner?

 

[Chad]

The reason I posted it is because I feel if one is inexperienced it's even more important to do research and not rely on a completely outmoded and arbitrary "rule". 
 
The term "fully stocked" is yet another excellent reason not to use the one inch rule. What it means to be fully stocked is different for each tank so if we could reword that, "A major benefit of this fishless cycling method is that you can now appropriately stock your tank in one go."

 

[daizeUK]

A beginner can easily research their fish but then they will wonder how many fish their tank will support and since this is different for each tank it's not something that can be researched easily.  Some kind of guess or estimate must be made.  A beginner might have no clue what would be an appropriate estimate so they would surely benefit from some kind of guideline, even if it is a little arbitrary.  Often the first kind of stocking advice they get is from their LFS which will often result in bad advice and overstocked tanks.  A general guideline helps to debunk this bad advice, when applied with caution and common sense.

 

[Chad]

A general guideline can help, as long as it's not 1 inch per gallon. That rule does nothing for swimming behavior, territoriality, aggression, etc.
 
If a person applies a little caution and common sense as you say then they don't need the one inch rule. I really see no place for it in the hobby especially in the Internet age where data, and advice on forums like this one, are so easily available. I would rather have a nice discussion with a beginner about possible options.
 
I love it when people post threads, "what about this fish?" We had one on Badis Badis today, another on stocking a 100L and of course as it relates to the OP's question on the subject. 
 
We'll just have to agree to disagree on this one because when I weigh the pros and cons of it the pros of thoughtful stocking win out over rule of thumb stocking any day.

 

[FlakeyBiscuit3.14]

 

If I were to stock the tank exactly as I wanted it, with no regard to this rule, I would probably put 6 pygmy corries, 4 julii cories, 8 bloodfin tetras, and 5 male guppies (from my 10G). 

One good site to get info on each variety of fish is Seriously Fish dot com.  You will quickly learn the Latin names of each as that is how they show up!
 
Then in going over things, you start to fine tune your selection.  I started thinking bloodfin tetras, then saw rummy nose tetras and fell in love, he heee.  So I have a nice school of 10 of those. 
 

I love the rummynose tetras too! But they seem more sensitive to pH and water chemistry than I feel I can provide as a beginner... I thought the bloodfins would be a good compromise
 
 
I've read through the debate going on daizeUK and tcamos, and I completely understand where you are both coming from. But I will say, as a complete newbie, despite all of the research I've done into individual species and their needs, I have been finding it much harder to determine the bioload capabilities for my tank. I know that this is determined by a number of factors, but having a guide to go off of would be extremely helpful. I just wish there was a more helpful guide/rule than the inch per gallon, which apparently is useless lol
 
If the inch per gallon rule is useless, how will I know when my tank is overstocked? By my ammonia or nitrate levels? 

 

[Chad]

If the inch per gallon rule is useless, how will I know when my tank is overstocked? By my ammonia or nitrate levels? 

I totally understand that rules are nice and make things easy. I'm sure all fishkeepers have at one time or the other.  One of the main reasons I've come to be against this particular generalization is because I've used it and it didn't serve me well. 
 
The bloodfin tetra you mention in your original post is a good example. On average they get about 2 inches long. By inch per gallon rule you could keep 5 of them in a 10 gallon tank. Problem is, a 10 gallon tank is too small for them. 
Then you have the issue you mention where the rule says you can only have 29 total inches of fish in your 29 gallon tank. That's of course not true. The pygmy cory is a great example of this. They get about an inch each but a person could easily keep 15 of them in a 10 gallon. According to the "rule" that's overstocking...but it's not really overstocking. 
 
Testing is part of the determining stocking levels but can't be relied on fully. A planted tank will do well taking up nutrients and leaving you with low testable levels but could still be overstocked. 
 
Territory is one of the main things to look out for. For example the dwarf puffer is about 1 inch full grown but requires a territory. In a well planted 29 gallon I had two males. In a 10 gallon those two males would likely have at it until one of the was killed. So you have to think about what the needs of the fish are. 
 
Your tank is generally divided into three zones: upper, middle, and bottom sometimes termed littoral, pelagic, and benthic zones though in an aquarium that's not a mirror of nature of course. Fish like cory are benthic, or bottom dwelling. They have a mouth that is on the bottom which makes them well adapted to foraging on the sand. Tetra have a mouth in the middle and tend to eat more what sinks in front of them or they hit the surface when they feel safe to do so and occasionally will pick something on the bottom. These two types of fish will generally stay out of each other's way so when stocking we can be more liberal with territory. 
 
Anyway...I think you get the idea. 
 
I think the best general rule for a beginner is to start out way understocked and don't give into the urge to buy every species you fall in love with. Keep just a few fish and enjoy the hobby for a while. A shoal of cory and another of a smaller tetra is a nice start for one tank. 

 

[daizeUK]

That's a good explanation tcamos.
 
I had a better experience with the inch-per-gallon rule for my first stocking.  I did my research first and decided on platies as a good fish that matched my water parameters and tank space.  The tank was species only which made the decision much easier.  Then I used the inch-per-gallon rule to give me a rough idea of how many platies I would be able to fit into the tank.  I found it to be a useful guideline when I had no other clue, although I will admit it quickly fell by the wayside when babies started appearing!
 
I would expect anyone to research their fish independently rather than taking the rule literally - e.g. "I can have 20 inches of ANY fish I like in my 20 gallon tank!" - but maybe some do!
 
I'll admit to a couple of problems when I applied the rule to my 32G tank.  I wanted to stock threadfin rainbowfish and harlequin rasbora in addition to a few male platies from my other tank.  Firstly I didn't realise at the time that threadfins and rasbora were both largely top-dwellers which was a mistake due to inexperience on my part.
 
On paper the threadfins and harlequins are both 4cm long so I worked out from the inches-per-gallon rule that I could keep a shoal of 8 of each in this tank.  However I subsequently observed that the threadfins have very slender bodies compared to the harlequins and I wonder how meaningful it is to compare them by length.  Although treated as the same size the harlequins probably have 2x or 3x the body mass of the threadfins, with a much larger appetite.
 
Currently I have 7 threadfins (I lost the harlequins in a heater accident) and am looking to increase the threadfin shoal size.  I'm thinking the inch-per-gallon rule is not serving me well in this instance and I could perhaps fit many more threadfins in the tank than the rule would suggest.  I also need to consider that the tank although 32G is only 2ft wide, but deep and broad.  I'd value your opinion on whether I've read the situation correctly - if the only tank occupants are threadfin rainbowfish and a few male platies, would a shoal of about 24 sound right?

 

[Chad]

 However I subsequently observed that the threadfins have very slender bodies compared to the harlequins and I wonder how meaningful it is to compare them by length.  

That's a great observation and one that often comes into play when dealing with deep keeled fish like angels or in marine fish like the bat fish or tangs. Per inch we would assume a full grown yellow tang would be doing well in a 55 gallon tank but of course the tang police would have you in cuffs for that! 
 
I'm still learning a lot about rainbow fish and haven't kept the threadfin myself so I would only be guessing as to shoal size.

 

[daizeUK]

Thanks Chad, no problem.  I'll start with another 8 to bump up the shoal (if I can source some nice females), then observe and apply common sense to any more additions!

 

[Chad]

Should be very attractive really.