The 1 Inch of Fish Per Gallon of Water Rule

As a frequent browser of several fish and aquarium forums, I frequently come across many things that make me go “Huh?”. You’ll even see this one inch of fish per gallon of water rule (or guideline) advocated on some of the fish and aquarium websites on the net. Let’s talk about why this rule doesn’t hold water.

Size Matters (fish size)

While this rule may work for some of the smaller fish species available to us, it doesn’t work for the majority. You see, fish grow. Most of the fish we get from the local fish store are juveniles and if we go by their current size at purchase when using this rule, we’ll definitely have problems as they grow.

If you stock your 10 gallon tank with ten, 1 inch Oscars, you’ll soon need either some Rogaine, Excedrin, a new tank or all of the above. Oscars, for example, can grow to be over 12 inches as adults! So, if you were to provide a constant influx of fresh aquarium water to this 10 gallon tank and assuming they wouldn’t fight with each other (NOT!) and assume that they’d live to maturity, you’d have potentially 120 inches of fish in your 10 gallon tank. Just a tad overstocked.

I could see where you could use this rule for say, 5 – two inch neon tetras in a 10 gallon tank. This would still be overstocked in my opinion, even if you were to provide optimal water conditions. It’s definitely not as bad as the Oscar scenario. Look at the adult size of any potential fish purchases before buying to see if they’ll go well in your fish tank.

Consider Aquarium Shape

We also have to keep in mind that larger fish require more dissolved oxygen in the water than smaller fish. Large fish also produce larger amounts of waste. Let’s use a 10 gallon tank for example again. A 10 inch Oscar will produce much more waste than 10, 1 inch neon tetras. That same 10 inch Oscar will also need much more dissolved oxygen than those tetras. If you have a 10 gallon rectangular shaped tank it would be better than a 10 gallon tall hexagon shaped tank. The rectangular tank will provide more surface area for gas exchange to take place. Carbon dioxide will leave the water and Oxygen will enter the water at the surface of the tank. That’s why you’ll sometimes here folks advocating more surface agitation in heavily stocked tanks. This surface agitation facilitates gas exchange.

Fish Temperament Is Also Important

If you were to stock your 10 gallon fish tank with 3 neon tetras (about 6 total inches as adults) and 1 convict cichlid (about 4 inches as an adult) you’d be within the 10 inches recommended by this silly rule and you may think it’s ok to go out and get this combination. Well, as many a convict cichlid keepers would tell you, those 3 neon tetras probably won’t last long in that setup. If they’re not eaten, they may get pestered to death by this pugnacious cichlid. Although keeping only one in this tank may limit it’s aggression, I’d be willing to bet that those neons wouldn’t last a month in this tank.

These were just some of things I thought I’d throw out there for those that are using this rule to stock their tanks. Hopefully you can tell that it’s not the best rule to go by and that it really only works for the small fish species that stay SMALL. Even then though, why make more work for yourself with water changes in your maxed out tank? Stock on the light side and make less work for yourself. Your fish may behave completely differently as well having more space and less competition for aquarium foods.

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