Opinions: Fish Size And Tank Size

I don't think there is any rule for working out tank size. A lot of the recommendations come from personal experience of keeping the fish - but recommendations from people who study fish, not just keep them. Anyone can set up a website saying "I keep these fish in this tank and they are fine" when anyone who really knows about fish can say those fish are "surviving not thriving"

There is more to consider besides size.
Shoaling species need a bigger tank than the same sized solitary fish simply because you have to keep more of them. A single male betta can be kept in a tank as small as 20 litres, but a shoal of Boraras needs a bigger tank.
Fast swimming fish need a longer tank that sedate fish. For example, on size alone zebra danios are fine in 60 cm but because of their swimming type they need at least 100 cm with some experts recommending 120 cm.

The most reputable site, Seriously Fish, gives a minimum tank size, but someone who writes fish profiles for the site has made the comment that their sizes refer to species-only tanks, something I had not realised before. As soon as you add other species, the tank requirements are bigger than those given on SF.

I can't find the tank sizes on their site... could you post a link @fcmf ?

They updated their site in the past year or so. I just checked on the black ghost knifefish profile, as it was a fish I considered keeping a couple of years back, and in the prose that still has the information (at least 6ft long and 2.5ft wide) in the further information/care section. A lot of other fish don't seem to have tank size information.

I've always worked to the recommendations from the Community Creator here on the Think Fish site.
I think though, there was some discussion a while ago about the accuracy of the CC and how up-to-date the database is.
Anyway, it seems to have worked out OK for me.
 

We did run a number of updates to the CC recently, some profiles were amended as were some of the minimum recommendations around tank size and groups too.


Good to hear it's worked well for you 

Couldn't agree more with this... I much prefer the smaller species, it make me feel l like less of a zoo keeper with animals in a cage! I have 2cm 3cm and a couple of 4cm fish in my 220 litre. Only exception is the Goldeneye Chichlid male which is my centrepiece fish and reaches 7cm.  I doubt I will ever own anything bigger than this.

I’m pretty sure the rule of 1” per (UK)gallon started as a British thing. It was in all the old books, and was still well ‘in force’ when I started in the early 70s. For most 1” non-territorial fish it works quite well, for a standard shaped tank. The mistake is applying it to anything bigger. A 2” fish is 8x a 1” fish so the rule breaks down very quickly.

There was also a “rule” of 1” per 12 square inches of surface area (=12x 1” fish per square foot).

Here are a couple of articles which look at various alternatives: http://injaf.org/articles-guides/general-guides/understanding-fish-stocking-guides/ and https://www.practicalfishkeeping.co.uk/features/frequently-asked-questions-on-stocking-densities/

It's such a complex issue, and so much of the information available seems to be quite dated.
It is also part of human nature to lean towards the information that backs up what they think, e.g. @Sue  example of "forgetting" that particular information relates to pencil-shaped fish.

Looking at the links that @fcmf posted, my favourite bit from the PFK article was

"So stocking densities are an oversimplification of something more complex?

Yes. Exactly. The actual amount of fish a tank can support is determined by its carrying capacity, but calculating this is complicated and beyond the average fishkeeper."

There is so much to take into account when considering the welfare of fish, and so much of the information available can be contradictory. As @Hampalong  has noted, use the rules as a starting point for research.

My knowledge and experience is what I would describe as "limited", and so far I think I've been quite lucky to have the experience and information available here to make sense of information I find elsewhere. It has always been encouraged to stock based on adult fish size, and not to stock to full capacity, whilst also taking into account the nature of the fish.

It could be my imagination, but now there also seems to be more of a leaning towards what I think of as "creating environments" for fish. In general, people have become more aware of animals and their suffering, in all aspects of life. Limiting any suffering and/or harm, caring for the environment, and trying to do the right thing is encouraged. With regards to fish, magazines and online articles show biotopes, and the creation of environments that mimic those found in the wild. On top of that local fish shops provide quite a selection of plants, decor, etc. and on-line shopping can pretty much get you anything. With a bit of artistic licence (can a fish tell the difference between a natural cave, or half a coconut shell covered in moss? and does it care?) we can provide our fishy friends with what they need, within the confines of a tank. Fingers crossed we are keeping them happy, healthy, and entertained.

There is a great difference between "what bioload (how many fish) can this tank hold" and "what size tank does this fish species need"

Most newcomers to the hobby think just in terms of the first. But a properly stocked tank also takes into consideration the needs of every species planned for the tank. Hampalong's link explains this well.

A lot of people (not just newcomers) stock right up to ‘the limit’ aswell. Some even ask “am I fully stocked yet”, like they believe you’re supposed to fill the tanks with fish. It’ll hold 36” so let’s go and get 36” of fish. Some of these, some of these, one of these... will they all live together? Yes. Boom. Fish soup. Every one of them feeling crowded, intimidated, intruded, stressed, no quiet place to rest... then the diseases start...

Like @Littlefish has done with her little fish, give any fish it’s ideal environment and you see a very different fish. This includes space and fewer tankmates. Which means stocking less is a good thing, rather than a limitation, as some would see it.