Age Old Question: How many fish should I stock in my tank?

[Odd Duck]

If I remember right, I think Axelrod’s first book mentioned a recommended max of 2-3” of fish per gallon, but spent extensive discussion explaining that you must adjust for heavy bodied fish, round bodies (like discus - you can’t just count inches of length when the fish is very “tall”), thick bodies, heavy eaters, excess finnage like angels or Bettas and the like, and allowed some wiggle room for very slim bodied fish.  
 

You also have to consider the relative sensitivity of certain species.  Some fish are very intolerant of higher nitrate levels and some are crazy tolerant of it.  It’s not good for anybody, but they can adapt more than you might think.  You’re risking making them sick the harder you push their tolerance, though.
 

I think this rule evolved into the 1” rule to make it a bit more “idiot proof” so people would be less likely to try putting 5 adult angels in a 20 gallon tank.  It really has no basis in the reality of what you can do if you have loads of plants, slim, small fish, lots of filtration, do frequent water changes, or any number of other variables mentioned.  
 

if you’re paying close attention and are certain you can keep up with water changes and plant maintenance, you’d probably be amazed at what you can do.  Is it smart to push the boundaries?  Not really!  You’re just making life harder for yourself.  Give yourself some wiggle room and the fishkeeping life is a lot easier.

I’ve been keeping fish for a decades (since 1975) and had a fairly recently got back into fishkeeping because I rescued a pair of Jack Dempseys and they bred (twice 🤦🏻‍♀️).  If I was smarter, I probably would have disposed of the eggs, but I didn’t.  I ended up with 3 tanks full, and I do mean FULL, of Jack fry.  I didn’t even want to test their water since I knew I wouldn’t like the results.  But they happily ate their way through a ton of food and I had almost no losses!  Raised over 1400 of the little devils before I got the parents put into a divided tank.  I had a 55 gallon, a 46 gallon, and a 29 gallon full of fry.  Not even *close* to what any sane human would consider enough room to raise 1400 fry to sellable/rehoming age/size but I got it done with *loads* of filtration and water changes.  Would I ever recommend somebody do that on purpose?  NO WAY!!!!!  But I got away with it partly due to having a lot of fish keeping experience and just plain burying my head in the sand and ignoring water parameters since there really wasn’t anything else I could do.  Nobody got sick, no Ich ever, and I only lost about a dozen or so fry.

I list this only to give an example of what *can* be done with enough dedication and determination.  But I use this really as an example of what NOT to do.  It was WORK doing all those water changes!  Be smarter than I was and be kind to yourself!  Give yourself some wiggle room!  
 

The recent power outages and cold weather here in the Dallas, Texas area are a perfect example of why you should give yourself wiggle room.  People lost *hundreds* of fish!  If you don’t have a way to sustain your fish during an extended power outage, you need to give yourself more wiggle room.  Can you pack up all your fish into coolers and take them someplace warm?  Do you have a generator to run heaters, air pumps, filters?  There were people that had generators big enough to run their tanks but their houses were so cold their heaters couldn’t keep up even with extra heaters added to the tanks and the tanks swaddled with blankets.  Wiggle room!

[Rory Waliser]

Thanks for all the suggestions everyone! Here's the fish-less setup as of now if anyone was curious. Many more plants to come!

[Will Billy]

I sorta use the 1 inch rule in an augmented way. Hear me out tho lol. I know the 1 inch rule is wildly inaccurate. But i do use it as a baseline in an attempt to pre calculate my bio load. Understanding the flaws of the 1 inch rule and adjusting for that flaw gets me to a good ball park idea of what my bio load “should” look like. However thats all i use it for, is to help establish a sort of imaginary baseline, that i can change as my observations reveal whats really going on. For example my mystery snail has much bigger 💩 than what i thought he would leave, or that i use 2 hob filters and 2 sponge filters in a 55 gallon. In short yes the 1 inch rule is junk, but if you know what you are looking for it can be a helpful tool understanding bio loads. The flawed design of the 1 inch rule sorta showed me the difference in bio load from 20 ghost shrimp compared to 1, 4” golden dojo loach. The flaw forced me to observe my tank inhabitants and determine that 1 dojo without a doubt has more bio load than 50 ghost shrimp. 

[gardenman]

A lot depends on your filtration also. In aquaculture filters aren't typically sized based on the size of the tank/vat/pond, but on the amount of food that will be added daily, the protein content of the food, the water temperature, and more. The folks at bioaqua.vn have an extensive article featuring all kinds of math and calculations on sizing a biofilter for aquaculture that could be applied to aquariums. (If you're truly insane anyway.) More fish, bigger fish, require more food. More food makes more waste that requires more filtration. You'll find aquaculture tanks that are way, way more crowded than most aquariums, but the fish thrive. I watch a lot of videos about koi on YouTube and some of the Japanese koi houses are just packed with koi in what appears to be absurd concentrations, but they have the filtration to handle it. It ultimately comes down to water quality. If the fish aren't suffering and seem happy, and the water quality is okay (ish) then you're good. My fifty gallon tank is absurdly overcrowded, but the fish are thriving. (Granted it does have three filters running on it, so that helps. A big sponge filter, a moving bed biofilter, and a big canister filter.) Put those same fish in a clean, unfiltered tank, even one four times the size, and they'd be dead in a day or two. Filters matter.