Asian fish never meet South American ones in the wild, so when they do in an aquarium setting, you can imagine that their 'languages' will differ...

How welcoming is your aquarium to new arrivals? Acclimatisation of new fish should be about more than just floating the bag and mixing the water, says Nathan Hill.



We all know to acclimate our fish, right? Of course we do, it's the one thing that even non-keepers seem to understand – the fish need ‘floating in the bag’ for a bit before they can go in. We can bicker about the details, but ultimately that's all it boils down to. For an indeterminate amount of time, the fish require a transitional period that allows the temperature and chemistry of their transport water to match the parameters in the tank.

Okay, we agree on that bit. But how many of us ever socially acclimate our fishes? How many even thought of it until you just read it there? Settling fish within the infrastructure of a community is an entirely different proposition.

Elephantnose fish have the highest brain to bodyweight ratios of any vertebrate on Earth — including humans!

Fish are brighter than most of us credit them for. Their social interactions are sometimes so advanced that they border on rudimentary civilisation. Sticklebacks have a tit-for-tat strategy that involves pairing up to investigate threats and novel objects – forming and breaking alliances based on their partner’s performance. Groupers call on support from eels to help flush out dinner, using brainpower over brute force. Elephantnosed mormyrids have the highest brain to bodyweight ratios of any vertebrate on Earth. Always thought that humans had the biggest ratios? Nope, the honour goes to a fish. Admittedly, most of that brain is given over to detecting electrical charges, but I can still think of people that an Elephantnose would probably beat in a game of chess.

The list goes on. Catfish have killer memories, remembering feeding cues from farmers for a year after they were last used. Trout learn to press buttons for food, and retain that skill for months. Wrasses smash the shells of bivalves by using rock 'anvils'. Some species are mentally adept enough to make elaborate nests or bowers (we all remember 'that' BBC documentary of the pufferfish nest).

Fish can count. Fish can play. Fish can build up trust and break it. Fish can spatially map. Yeah, fish are pretty bright. Except the Bony-eared assfish (honest, that’s its name!), which boasts the smallest brain to bodyweight ratio of any vertebrate on the planet. Moron.

While two male Betta are pretty much guaranteed to fight in the aquarium, in their natural habitat they tend to be more choosy about which rival to take on.

What of fishes’ social lives? Betta splendens Fighting fish are a good example of (anti)social behaviour taken to extremes, and anyone who keeps an aquarium will be able to tell you that they live up to their names. They fight, brutally, relentlessly to the death, as though it’s their raison d’etre. Yet this detracts from their deep, cognitive sides. Young male and subordinate Fighters will watch altercations between rival males, as will curious females. Victories and losses are remembered, and when it comes to an individual pitching himself within the fighting ranks, a new Betta on the block will carefully consider which fights to pick and avoid, based on what he has seen.

Cichlid keepers will understand the impacts of adding fish willy-nilly to a set up. Malawian species live in tight territorial boundaries, establishing who is who in their miniature world, and guarding their property with vigour.

Sweet and innocent — or just waiting until you go out of the room before they give their tank mates a hiding?

Cichlids can be underhand and crafty. I've experienced the Jekyll and Hyde nature of Angelfish in aquaria. Whenever I was about, they were all cuteness and simpering. When I was out of sight, peering at the tank through a crack in the door, they were hellish, systematically chasing down tank mates and hammering them.

But then, who can blame them? The aquarium is an unfamiliar environment for fish used to rivers, ponds, lakes and seas. It's like taking the contents of a forest – the bears with their wide ranges, the roaming wolves, the deer, the hares, and cramming them all into a single acre of space. Interactions with each other are confused, and fights are going to happen.

Dissect the stocking of most aquaria, and the list of ‘ingredients’ reads like an exotic recipe. We often dangerously assume that a fish is a fish is a fish, when the best biologists can’t even decide what constitutes a ‘fish’ in the first place. We make assumptions about behaviour, failing to factor in that the definition is so broad that some fish are more closely related to we humans than they are to other fish. Go figure.

Things are compounded when fish from all around the world are being tossed together. Asian fish never meet South American ones in the wild, so when they do in an aquarium setting, you can imagine that their 'languages' will differ – think of travelling somewhere like Mongolia and not meeting any other ex-pats, and you’ll get the idea. Customs will vary. What we find acceptable might be frowned upon, and vice versa.

In aquaria, Asian barbs have their own way of interacting with shoals of different fish. But the chances are that a shoal of Amazonian tetra won't understand the rules – they've evolved their own ways of fitting in. In some cases, an aquarium is little more than a huge cultural time bomb, waiting to blow.

Adding fish to a tank warrants more than just a 'toss them in, hope for the best' attitude, which is what happens currently. I often hear retailers being asked to take fish back in, because they don't fit the dynamic of a customer's set up. They might be spooking things, or being bullied. Either way, it might be time we addressed this as an issue to be prevented, rather than cured only when the symptoms start to show.

A cursory search tosses up all sorts of data about the stress involved in bad acclimation. The first hit I get from a scientific journal cites stress responses from poor social acclimation, increased cortisol levels, reduced numbers of circulating lymphocites, signs of chronic stress, bacterial infections and growth inhibition. And that was just in the abstract.

So how? And which species?

Let’s start with the most obvious. Shoaling fish. I’ve written at length about shoals before but they present the greatest area of substandard welfare in the hobby, to my mind. The key factor is numbers, as nobody seems to put together large enough shoals.

Give shoals of rainbowfish plenty of room to stretch their fins. You don't have to go this big, but a group of half a dozen Red rainbows is unlikely to look its best in a three-footer!

A large shoal isn’t always a welcoming place, especially a long-established one. Moreover, a shoal will develop its own internal dynamic, and pending species this may be aggressively expressed to newcomers. Contrary to what I just alluded to above, it’s not necessarily about big shoals, but about appropriate shoals. Rainbowfish, by way of example, will likely fare worse at high densities in a smallish aquarium. The males, with their need to display and subjugate, will struggle to police their tank mates, to the detriment of all.

Adding fish to existing shoals can be intimidating for the newcomers, so ensure there is plenty of cover available until they gain confidence.

For most shoaling types, it would appear the best route to socially inject new fish is to provide plenty of safe spaces, and let nature take its course. Dense planting – especially feathery leaves that a fish can really bury itself in – offers an area of refuge. Myriophyllum or Lymnophila are fluffy choices that fit the bill.

For the likes of Rainbowfish, visual and olfactory cues can help to smooth any introductions. A clear or mesh divider can be used to fence off a portion of tank, allowing fish to see the new arrivals, but not reach them. Potentially, the resident fish will become increasingly accustomed to the newcomers, perceiving them as less of an intruder and more of the ‘furniture’ of the tank, so that after a period the divider can be removed.

Many cichlids would benefit from similar, though the ongoing trend is to add and watch, and remove the new fish if it gets too much of a battering – I’m sure I can’t be the only one who doesn’t feel comfy with that.

The obvious option is to keep one fish in a breeding trap (or something a little bigger, like a submerged faunarium) inside the main tank so that other fish can get used to the smell and sight of it. If you were clever, you'd incorporate rocks into this faunarium and place it somewhere to denote a ‘territory’, so the resident fish would come to recognise the area as claimed when the fish is eventually released (along with the rock).

If you were ultra savvy, you might even opt for a gradual introduction. If the newcomer is small, place an aperture in to the faunarium which only that fish can fit through. This way it can begin to venture out, but dash back inside its ‘safe zone’ when confronted. Again, a great way of allowing for gradual introduction, and allowing fish to rewrite their territories. The next stage would be to have the faunarium in place, but with the lid removed. After that, remove it altogether.

There are social acclimation boxes made for this purpose, though primarily aimed at the marine market. Personally, I see benefit in making a box of your own, utilising a clean margarine or ice cream tub, and either affixing suckers or a clamp to keep it in place. The benefit here is that I have little concern for cutting holes in a cheap tub like that.

A big problem is the lack of research in this area. There are so many fish that it would be an almost infinite task to study the compatibility of each species with each and every other. Fish themselves have such broad temperaments, even within species, as to render such an exercise of use only on the broadest scale. Not that any of this is an excuse for an aquarist not to try, at least. For a while, we have already considered the social consequences of some communities. Marine keepers in particular have considered the roles that fish play in the coral reef ‘cities’ they try to replicate at home. Factors of mutualism, such as with cleaning fish, are now embraced at length. In the freshwater hobby, only ‘trial and error’ cases found from forum anecdotes would amount to any kind of quantitative study. We are still in the early days, but I do hope that we can start to consider the importance of furthering our knowledge in this area.

Don't terrify small, sensitive pencilfish by sticking them in tanks with large and boisterous species. It won't end well.

In all cases of social introduction, the responsible aquarist should strive to understand the fish they’re adding. Granted, most newcomers don’t even know which fish they want before going out to buy them, so the majority of my concern is wasted energy. But for you, the ethical-minded, well-read and passionate fishkeeper, there are considerations you should try to incorporate, and these include:

Is your fish a known prey fish (such as Nannostomus) and likely to be terrified of anything larger than itself?

Does your fish naturally form tight or loose shoals?

Is your fish known to have a competitive hierarchy within a shoal? Barbs and rainbowfish, as well as some tetra are culprits here.

Does your proposed mix of fish include bold and shy species, and will these clash?

Will it be possible to rearrange the tank layout to redraw territorial lines? This can work particularly well with certain cichlids, but can also be the trigger for a genocidal land grab, and can create more problems than it resolves.

Are there variances in aggression depending on gender?

Will gender ratios increase or decrease the likelihood of aggression overspilling to tank mates?

Don't try to keep guppies with male Fighters. These mild-mannered livebearers won't stand a chance when the Betta mistakes them for a rival.

Are appearance similarities enough to trigger an aggressive confrontation between species (think Betta splendens mixed with flowing-finned guppies)?

Can territory-oriented species be added last to a set-up? This may help them to accept existing fish as non-intruders. Note that this only works occasionally.

Most important of all – do you have a safety net in the event of an altercation? If you are planning on putting fish together with a high risk of incompatibility, then it’s down to you to ensure that you’ve an alternative set up to isolate one of the fish. If it all kicks off, you have an ethical imperative to resolve things – those fish never asked to be put together in a boxing ring.

Do you have a plan of introduction? If you anticipate troubles, is it even worth the potential stress (to fish, not you) of forcing a union? Remember, if your ethos is ‘add it and hope for the best’ then you might benefit from five minutes of introspection. If you still can’t see what’s wrong with that mindset after five minutes… give it another five minutes.