Confused About The Inch Per Gallon Rule

[hillmar77]

I'm just a little confused about the inch per gallon rule! I'm currently overstocked on my 10 gallon tank setup but it seems if you keep smaller fishes, the rule can be bent! The question I have is why does it take a 75 gallon minimum to stock a single 12 inch fish? For example.... Tiger Oscar from what I read require a minimum 75 gallon tank and with 2 you need over 100 gallons. I mean there seems to be plenty of room for a Oscar to swim around in a 50 gallon tank, and I know they aren't hyper fishes compared to say zebra daino's are. I know they produce huge amount of waste but you can always take care of that with massive filtration. Just asking since I'm setting up my 55 gallon ATM.

 

[Tolak]

The rule is not a rule, it's a guideline designed to keep beginning fishkeepers out of trouble. The entire guidline is;

One inch of slim bodied fish, that grows to no larger than 3 inches, per gallon of water.

This obviously does not encompass larger cichlids such as oscars. For larger fish you really have to go by the individual species, as some need more territory than others. The general guideline for larger fish is 4 times the adult length one direction, 2 times the adult length the other direction for the tank footprint.

Remember that just like the inch per gallon deal, this is only a guideline. This is why a 75 gallon, which is 4'x1.5' is considered bare minimum for a fish that most certainly will get larger than 12". Inch per gallon is a beginner's guideline, with experience many fishkeepers stock at double that rate or more.

 

[Assaye]

The inch per gallon rule isn't a hard and fast rule. It's basically there to give a vague guide to how much fish it is safe to put into a tank if you are a newbie with moderate filtration and fairly easy going community species.

As you have noted, it can be bent in many cases, such as with lots of filtration, lots of plants, certain fish that need to be packed in (like some cichlids) and small peaceful fish.

It's essentially there to stop someone putting 20 guppies in a 5 gallon tank, or something else stupid.

However, to address your question of the oscar - a 12 inch long fish needs a large tank for two reasons. Firstly, there needs to be plenty of water to help dilute any pollutants before the filter can deal with them. Secondly, they do need a lot of swimming space! A 40 gallon would be 3-4 foot, which would only give a 12 inch fish 3-4 times it's own body length to pace around, and very little room to swim up and down or turn around. It would literally be like you living in your shower-room or walk-in closet. Adittionally, they need lots of space to stop them getting stunted as they grow up. I agree that an oscar needs an absolute minimum of 75 gallons and I would personally not want to put an oscar in less than 100 gallons.

I personally hold a rule that says give a fish at least 6 x it's own body length, ideally more. So I'd want to see fish like oscars, bala sharks and clown loaches in 6 foot tanks.

This rule can also be added to the inch per gallon rule and you get a good guideline for how big a tank needs to be for your fish.

I will add that there are two "stock caps" on a tank. One is a soft cap - the filtration cap. If you add more filtration, you can go over that cap. The second is a hard cap - the size of the tank and the needs of the fish in it. Too many fish can and will cause stress and make the fish much more likely to fight and get sick. This cap on the stock shouldn't really be exceeded.

 

[Katty]

Agree with the top two posts completely, but here's the visual take on it:

A 12" fish in a standard 55g tank. The picture is also to scale (5 pixels = 1 inch)

 

[waterdrop]

Agree with Tolak and Assaye... and let's all try to keep encouraging that the word guideline always be used in place of the word rule. Beginners really have little idea what they're getting in to with full and overstocking. It becomes significantly more difficult to maintain the tank to the same healthy standards and the worst I think is when the emergencies, like power outages, come.

~~waterdrop~~

 

[OldMan47]

The inch per gallon guidance is meant to keep people new to the hobby from overstocking their tanks too much. It is quite useful in fulfilling that role but that is about all that you can say for it. If you keep mostly fish the size of platies or even swordtails, it will place you in the position of a slightly understocked tank, which is where I like to land. On the other hand, if you have only one tank and can afford to give it the needed attention, a 1.5 to 2 inch per gallon stocking can be maintained with regular enough water changes. I bend the inch per gallon rule to the breaking point on my small tanks, like my 10 gallon ones. Those tanks do fine with far more fish than the inch per gallon guidance would dictate. Please keep in mind that I am an experienced fish keeper and I do not hesitate to do a 90% water change when it is called for by the way my fish are acting. That is a big deal when you compare my stocking practices with those of a new person. That new person will hold back and seek some form of confirmation about what they should do with respect to water changes while an experienced fish keeper will do the water change and ask if they did the right thing later. There is a huge difference between these apparently close attitudes. None of the experienced fish keepers will hesitate for a moment before doing what a new fish keeper would consider a threateningly large water change. Instead our experience will tell us the fish do far better if we err on the side of more and larger water changes. It is a reality thing for me, your experience and the advice you have received may make yo cautious about doing a water change while my own experience often makes me ask "why do you hesitate, get on with it" when faced with a water change question. Fresh water is your best friend in any fresh water tank and the modern dechlorinators make using fresh tap water a thing for almost no concern.