What, actually, is Stocking Levels?

[Bradders]

Since my re-engagement with aquariums nine months ago, I have read a lot about this topic, which is actually a hard one to quantify. As a result, I think it's worthy of a focused discussion amongst the community.

It can also be quite touchy. There seems to be no "math" around it, but a lot of people have differing views on what is right and what is wrong. And some of that is passionately debated. We have a great collection of members here, so I wondered if we could debate this in an open and non-confrontational way. Just to bring in all the experienced fish-keepers, their views and thoughts.

I mentioned the touchy side. And I don't think I should start this thread without my own disclosure. I have a small 100-litre semi-planted aquarium with 33 fish - which equates to 49 fish inches. Mollys, Guppys, Platys and Tetras all live in that space. That is 3 fish per two gallons (~10 litres) of water. I can find sources to say that is perfectly OK, and others to say that I should be imprisoned. It's not an entirely comfortable position to be in when you are trying to do the best for your pets - and keep a thriving and beautiful community.

To add to this, you have an aspect of how (perceivably) happy the fish are. And to demonstrate this, I have 5 platys which were introduced early on (when stocking was low and space was maximised) and squabbled. They just love to squabble! The fish load has increased since then, and all of them get on and swim around happily - but the platys continue to squabble like they did when more space was there. So what does that mean? Should I have stopped with the 5 Platys, as it was perceived that the space was too small and resulted in squabbling? Or was I right to carry on expanding the livestock with the evidence that all other fish seem settled and the Platys are still squabbling?

I would love to hear about your experiences and views. I would invite people to be open and not critical but to contribute to a debate that has plagued fish owners for, seemingly, many years.

Over to you!

 

[tam]

There is no maths because there are too many variables. You could have a long shallow tank with lots of length to swim or a tall narrow tank that a larger fish couldn't turn around in and they'd both be the same volume. Tank length effects swimming area which is relevant in fish activity levels - a fast fish needs length to avoid hitting the glass if they panic or get frisky. Surface area effects oxygen levels, a deeper tank, shorter tank has smaller surface area. Fish length is a very rough indicator of waste production e.g. a 4" kuli loach will produce much less waste than a four inch goldfish.

Really it's something you have to consider the specific circumstances so fish species, tank volume/shape, filtration/plant mass, maintainance you can commit to, even things like how hot your house gets in summer i.e. push your stocking to the limit and you have no wiggle room so temp goes up in summer and you run out of oxygen where as a low stocked tank would be fine.

 

[fredi]

Imho it very much depends of the fish you intend to keep, rift valley cichlids require more dense stocking than most, you then require adequate filtration, (and little in the way of plants to help)
I believe it was Churchill who said that rules were for the guidance of men and the obedience of fools

 

[simon_the_plant_nerd]

My aquarium volume is similar to yours and I have a total of 34 fish. If we do the “fish-inches” metric then I’m at about 42” when everything is fully grown (nowhere near that yet). All the fish seem happy and healthy.

I had much more problems with fish fighting and not eating when the tank was more empty. As I added more plants, wood and fish it became calmer. If any of my fish outgrew the tank or became a problem I would move them along.

I don’t know what the “right” answer is but I’m happy that I’m keeping the animals welfare high.

 

[Maf 2500]

Assuming that they have the same general body shape and the same metabolic rate, a single four inch fish would have the same bio load on the aquarium as 64 one inch fish. Because it would be an equivalent mass (weight) of fish producing an equivalent amount of poop and other excretions.

So fish inches is not a very valid metric...

 

[simon_the_plant_nerd]

So fish inches is not a very valid metric...

It’s like “watts per gallon” in lighting. Only works if everything else is kept the same.

 

[fredi]

It’s like “watts per gallon” in lighting. Only works if everything else is kept the same.

Watts per gallon is (imho) these days, not a useful metric
A t5 fluorescent tube outputs a very different lumen per watt compared to a modern led light

 

[Tim Harrison]

So fish inches is not a very valid metric...

 

[MichaelJ]

To me the most important metrics are free swimming space, the size of individual fish sp., and the natural behavior characteristics of the species you keep. If you tank is very densely planted with lots of hardscape for instance you're going to be limited in terms of the size and activity level of the fish rather than some absolute number of individuals relative to tank size.

As an example, in one of my 150L tanks I keep a fairly large pair of Anglefish, a small school of Rummy nose tetras and a few Bleeding heart tetras. The swimming space for the Angels is definitely limited to the point now where it's like I am keeping the Angels in a 40-50 L tank - too small. Likewise, the Rummy nose tetras are hyper active and they could need more space too. However, if I would apply some length-metric to this tank I would easily reach the wrong conclusion that the tank is lightly stocked and all should be fine.

An inappropriately stocked tank is very much like obscenity... you know it when when you see it.

Cheers,
Michael

 

[LMuhlen]

I see it like this, stocking levels are to be compared to filtration capability. Keeping a lot of fishes requires very good filtration, be it a traditional filter, or plants, or constant water change, or any combination. The sizes of the fishes are only important for their relation with the amount of pollution they generate.

Now the size of the tank relates mostly to the behavior of the fishes. Fast swimming fishes, or fishes that require their own territory, need larger tanks, while calm slow swimming fishes can be kept in smaller tanks. The behavior may depend on the number of fishes in the tank and there could be a point where even small schooling fishes feel overcrowded, but I find that is rarely the case. And two territorial fishes may require much more space than one territorial fish sharing the tank with non territorial ones. A large fish needs room to swim and move around, even if it is a slow swimmer, but two of those fishes can probably share the same space as one.

And then there is the aesthetic side of things, some people like to see a tank where they need to look for the fishes, while others like to have them present all around. No rules here, to each their own.

 

[hypnogogia]

Assuming that they have the same general body shape and the same metabolic rate, a single four inch fish would have the same bio load on the aquarium as 64 one inch fish.

Really?

 

[Bradders]

That sounds about right.

Here is an Oase 250 bio load capability (standard manufacturer-supplied media) based on fish size. You will notice that it can support 168 x 1-inch fish, or 4 x 4-inch fish.

 

[simon_the_plant_nerd]

Watts per gallon is (imho) these days, not a useful metric
A t5 fluorescent tube outputs a very different lumen per watt compared to a modern led light

Agreed. It is basically useless. I started a post back in January pondering lighting. I didn’t get very far to be honest and I’m still just guessing as far as my lighting goes.

 

[MichaelJ]

That sounds about right.

Here is an Oase 250 bio load capability (standard manufacturer-supplied media) based on fish size. You will notice that it can support 168 x 1-inch fish, or 4 x 4-inch fish.

View attachment 217695

None of this is directed towards you @Bradders ... you are raising a legit and interesting topic 👍. (I assume you're not secretly working for the marketing dept at OASE though - yes, they make good canisters for sure... many great hobbyists around here are using them. ).

Without doing any regression analysis on this, it look suspiciously like some approximate near-cube law that attempt to guesstimate the bio-load from a certain amount of fish vs. their size (body volume)... The chart by itself is totally bunk IMO. How did OASE come up with this guide I wonder.... Is this for a tank without plants or substrate, or perhaps some plant or.... ?? How frequently is water assumed to be changed... weekly, monthly, never ... and how much ? Are the "fish" carnivores, herbivores or omnivores. What and how much are they fed? We could go on and on...

Well, putting up a guideline as OASE is trying here is tricky... even solely from a bio load perspective this chart is meaningless without factoring in specific conditions. And as both @LMuhlen, myself and others have touched on, stocking level is much more than bio load. And also, given we are a plant centric forum, our substrate and plants provide way more bio filtration than our filters. ... many of us, myself included, are not even using any artificial bio filtration.

Cheers,
Michael

 

[fredi]

None of this directed towards you @Bradders ... you are raising a legit and interesting topic 👍. (I assume your not secretly working for the marketing dept at OASE though - yes, they make good canisters for sure... many great hobbyists around here are using them. ).

Without doing any regression analysis on this, it look suspiciously like some approximate near-cube law that attempt to guesstimate the bio-load from a certain amount of fish vs. their size (volume)... The chart by itself is totally bunk IMO. How did OASE come up with this guide I wonder.... Is this for a tank without plants or substrate, or perhaps some plant or.... ?? How frequently is water assumed to be changed... weekly, monthly, never ... and how much ? Are the "fish" carnivores, herbivores or omnivores. What and how much are they feed? We could go on and on...

Well, putting up guidelines as OASE are trying here is tricky... even solely from a bio load perspective this are almost meaningless. As both @LMuhlen, myself and others have touched on, stocking level is much more than bio load. And also, given we are a plant centric forum, our substrate and plants provide way more bio filtration than our filters. ... many of us, myself included, are not even using any artificial bio filtration.

Cheers,
Michael

Have to say that I agree with this
Experience tells me that a 10” Oscar, requires many times (much, much more than double), the filtration capacity of 10 x1” tetras or guppies
Imho inches of fish to size of tank guesstimating is not useful
As has been mentioned above, it’s the combination of tank volume, and the capacity of the filtration system (inc plant mass) that determines the number/size/type of fish that can be sensibly accommodated

 

[MichaelJ]

Experience tells me that a 10” Oscar, requires many times (much, much more than double), the filtration capacity of 10 x1” tetras or guppies

Definitely. But keep in mind that OASE is actually trying to factor in body mass/volume... The OASE math above is roughly length raised to the power of 2.6 with some offset and scale - so the bio load of one 10" Oscar is roughly 50 times that of 10 1" tetras... which seems crazy unless you actually have kept large Cichlids and then you know its not

Cheers,
Michael

 

[fredi]

Definitely. But keep in mind that OASE is actually trying to factor in body volume... The OASE math above is roughly length raised to the power of 2.6 with some offset - so the bio load of one 10" Oscar is roughly 50 times that of 10 1" tetras... which seems crazy unless you actually have kept large Cichlids and then you know its not

Cheers,
Michael

👍
Filter manufacturers sell filters, that’s what they do 😂😂

 

[Maf 2500]

Really?

Pretty much. It is 4 times the length, 4 times the width and 4 times the height, so 64 times as big and producing 64 times the pollution. In practice it will not scale absolutely as smaller organisms tend to have a higher metabolic rate than larger ones, which I think is why the Oase chart posted above has a lesser ratio. Basically each time you double the length of a fish the bioload increases by 8 times minus a small reduction to account for differences in metabolism (if any).

And obviously I agree with all the points above about swimming space, lines of sight, aggression, hiding places, plants and surface area and so on. Bioload is not the only factor to consider, but it is an important one.

 

[Bradders]

None of this is directed towards you @Bradders ... you are raising a legit and interesting topic 👍. (I assume you're not secretly working for the marketing dept at OASE though - yes, they make good canisters for sure... many great hobbyists around here are using them. ).

Fear not! On both counts, I am not guilty! I love debating on a topic and don't work for Oase. (Honest!)

However, I must confess that I may have misled unintentionally. That chart is not from Oase; it's from a study that was then discussed on aquarium science. Some notes:

• The study solely focused on biofiltration capacity within a filter. It did not consider other aspects as mentioned above. i.e. plants.
• The article was based on the metabolic weight of fish and waste production, and therefore the loading that would place on the filtration system.
• The article states that the data are approximations and indicative but does demonstrate that metabolic weight has a direct impact on stocking size ratios.

Experience tells me that a 10” Oscar, requires many times (much, much more than double), the filtration capacity of 10 x1” tetras or guppies

I think that is what the chart is saying. In the table, you can see (based on the approximations and looking at filter-based filtration only) that you can keep 168 x 1-inch fish due to the metabolic weight and waste output. But as you move toward a 6-inch fish, you can pretty much only keep 1 with the same filter.

 

[MichaelJ]

However, I must confess that I may have misled unintentionally. That chart is not from Oase; it's from a study that was then discussed on aquarium science. Some notes:

Ok, then. Well, Aquarium Science got a lot of fine information and advice if your running a plant-less or near plant-less tank... They are putting an overt large amount of attention on external filtration which matters far less in a moderately to densely planted tank. Flow and nutrient distribution matters a lot of course in a planted tank, external bio filtration not so much.

Cheers,
Michael