13. Stocking

@Phil and @Dave

I have thought about this question before, and one hypothesis I came to is that the 1 inch per gallon rule might actually have something to do with the species of fish popular in whichever country/culture where that rule first arose. The inch/gallon rule is baseless if one takes the tank size to fish weight ratio as the basis for calculating density. However, it actually is a good rule of thumb when dealing with certain fish species that exhibit territorial behaviour.

I like to think about it this way: some fish are like (human) teenagers. If you make 2 or more teenage siblings share 1 bedroom, they WILL squabble sooner or later. The smaller the bedroom, the worse it will be.

So, I would investigate the ethology of the fish species popular in the country/countries where those guides that first mentioned the rule were published. By the way, I’m from Asia, and generally stocking densities are WAAAAY higher than the guidelines promulgated as gospel on the (English-speaking/American-dominated) internet. Of course, in the myths of fishkeeping, the European literature matters a lot too. But my point is, the sociocultural aspect of it all matters.

The cynical view would be that tank manufacturers wanted to sell more tanks.

It could also be that technology was the “culprit”. One inch per gallon might be a sensible guidelines in a time where aeration and filter technology were nothing like today’s.

Just some ideas on how I would approach the question. I have thought of writing a book on the history of fishkeeping before, but I shelved that idea because the volume of archival (and other) work needed isn’t something I see as practical unless some foundation funds it. But I do urge anyone researching the topic to please not publish anything as fact unless one is very confident in their conclusions. The available historical materials on this topic are already muddy enough. Muddying them further would make serious work very tough for future historians.

But, lest anyone get the wrong idea, I hasten to add that that the hypotheses above DO NOT conflict with the fish weight/tank volume paradigm of stocking density. To use the same analogy. If 2 teenage siblings are sharing a bedroom of a given size X, and you hire a contractor to install a partition and turn that bedroom into two, peace returns. (This analogy is valid because fish, unlike teenagers, do not leave their tanks and roam the house.)

Expanding upon this, the metabolic weight to tank volume (or effective filter surface area rule) really should be seen as just one of several possible limiting factors in stocking. Aggression is the obvious alternative that comes to mind. You cannot stock multiple male bettas in one tank based on their metabolic weight.

Another issue with the science behind stocking densities is that the formal scientific research is mostly based on food fish – and aquaculture operations DO NOT do research on fish that people don’t eat. Also, one of the criteria for what makes a “good candidate” for aquaculture is whether the fish can be reared densely. So, there is an inherent bias in the journal literature when it comes to stocking densities.

Finally, even among freshwater food fish, the literature is heavily biased towards the tilapia – the classic model organism. But what if you have food fish that breathe air? The snakehead is one of them. So the limiting factor wrt tilapia (DO levels) might not be the limiting factor for snakeheads. But since the majority of research funding is disbursed in North America, and tilapia sells better in that market, the literature would again be inherently biased.

Just my two cents.

Hi Dave, right now I have 2 corys and a baby ramshorn snail in a heavily planted 10 gallon with a HOB filter with a lot of filter floss as the filter media. I plan to add another cory, 4 Kuhli loaches and somewhere between 5-6 of either neon tetras or white cloud mountain minnows. Will the tank be able to support this kind of bioload? the tank has been cycled for 6 months now.

In reply to dave, thanks for taking the time to help me out.

It perfectly captures my thoughts when I read this article. Many aquarium articles from Europe and the U.S. tend to present fixed numbers for fish stocking, and anyone who keeps fish at a higher density is often criticized. In Asia—especially Southeast Asia—we have a different approach. We usually don’t use sponge filters or canister filters; we use sumps that are as large as the main tanks. The water pumps have a very high turnover rate, and when using moving bed media (e.g., K1), we employ strong air pumps.

I’ve seen tanks around 160 liters (about 40 gallons) that house more than 30 large ranchu and oranda goldfish. The water remains crystal clear, odorless, and the fish are healthy. When it comes to koi, filtration systems in Asia are often half the size of the pond itself, and some even build filter chambers equal to two-thirds of the pond’s volume. That’s why seeing large koi crowded together in clean, clear water is quite common here.

Even with discus fish—known for being extremely sensitive to water quality—Southeast Asian breeders are masters at developing new, colorful strains. Our discus tanks are stocked very densely, yet the fish stay strong and healthy. Therefore, stocking density primarily depends on the filtration system. What may seem overcrowded and risky for one person could be perfectly fine for another. It doesn’t mean that the latter doesn’t care for their fish or doesn’t know what they’re doing.