Back in fashion: Fish-Only Saltwater Tanks

Dated, gaudy, lacking in taste, not challenging, not natural – we put the case forward for fish-only marine aquariums, and prove that they can be anything but.


If someone tells you they have a marine tank these days, you’ll almost always assume that it’s a reef tank. After all, the selection of corals available to buy is better than ever and because of the wealth of information, experience and knowledge, you’ve a better chance of succeeding than ever. But if you’ve been there, done that, and got the Iquatics “I’m a fish geek” T-shirt, I proposition you to consider a new challenge – the fish-only marine tank.


The olden days

So fish-only usually conjures up that stereotypical picture, one of a long, narrow tank with a few bleached coral skeletons and the token predators like a Lionfish, Panther grouper and Moray eel. The sand and skeletons have a light brown cyano film on them to match the fake wood grain trim and the whole thing is, well, a bit bland.

Fast forward 30 years and fish-only marine tanks have even been on TV, courtesy of Nat Geo’s Fish Tank Kings and Animal Planet’s Tanked on Sky. These American shows portrayed a different kind of fish-only marine tank across the Atlantic. It was like they’d  been turboed, nitrous injected and technicolored for the 21st century aquarium owner. The properties of acrylic aquariums are allowing more and more lavish shapes, sizes and designs, proving the “if you can dream it, we can build it” slogan of Tanked’s Acrylic Tank Manufacturing owners Brett Raymer and Wayde King.

These mega marine tanks are anything but bland. They’re often themed towards their celebrity owners, sports personalities or tv personalities, and professionals like dentists who work with teeth, so need an aquarium with fish with teeth, like sharks. Or shop owners who want their aquarium to be in the shape of the product they sell, like a lava lamp, or a smoothie.

Rightly or wrongly, these tanks have raised the public profile of aquariums and fishkeeping, and fish-only marine tanks. And I even admit to watching them, because it was fish on tv, and if fish are on tv, I’ll be watching them.


Fresh ideas

But the modern fish-only doesn’t have to be any of the above, and I urge you instead of writing one off as being too easy or a bit naff, or even unnatural, to see it as an opportunity – a new challenge – and one that if you pull off properly could well and truly cement you into the cool camp with many a seasoned reefkeeper.

Go onto an online reef blog of late and I’ll bet 50% of the chatter will be on new and rare fish. Deep water butterflies and hybrid angelfish top the charts as the most expensive marine fish available, but you won’t be putting them into a reef tank any time soon as they will nail any invertebrate flesh in no time. Also you’re going to struggle to recreate anything deepwater-like in the average reef tank due to your lighting needing to be of the sort that encourages photosynthetic algae in coral tissues. Going through the rigmarole of decompression and two feet deep verses 200ft deep is one thing, but spending the rest of your days under simulated burning midday sun is quite another. 

So what do you do? You set up a fish-only and start collecting. But if you’re going to spend upwards of £50 on a fish or even £5000, you’re also going to need to do your damnedest to keep the thing alive, and that means medicating with a copper based medication. And copper isn’t reef safe either.


And why let the freshwater guys have all the fun when it comes to aquascaping when you can do it too – in a fish only. Some aquascapers are just born artisans, but one major factor which helps them along their creative way is the ability to aquascape with rocks and substrates when the tank is dry, and taking days or even weeks to achieve the best look before filling with water. Buy some reef bones, reef plates or any of the myriad of dry, replica live rock and you can get creative, forming saltwater versions of the planted tanks’ Iwagumi, or get your acrylic rods and epoxy out to form gravity defying overhangs, caves and bridges. One of the challenges with a reef tank is that your hardscape grows, moving your focal point from that perfect two thirds golden ratio placement over time, but do it all dry, take your time, and go fish-only and your hardscape will always stay exactly where you placed it, enhancing your choice of fish and the aquascape.

Google fish-only tanks and most still look pretty poor, but it’s not the fault of the fish themselves, its the fault of the person who aquascaped it. My advice is to get friendly with a local aquascaper, give them a car boot full of rocks and sand, and an afternoon, and I reckon they’ll create something looking pretty legendary in no time.


Fake corals

Don’t hate me, but I’m not against fake corals. They’re getting better than ever in the looks and materials department and the fish don’t mind one bit. They provide cover and again, if they’re placed right can looking really striking. Use the rules of two thirds, just like in freshwater aquascaping and make that large Acropora or Montipora jut out so its the one you look at first when you view the tank. 

Buy multiples of one type and one color of fake coral, only in different sizes, and group them together. Do that and they’ll start to look natural. They can be expensive initially, but you’ll only buy once and the best ones can even be removed and washed in the dishwasher. 

Fake corals can also offer biotope opportunities. They’re your only option if you want to create a Caribbean biotope with all those sponges, gorgonians and elkhorn coral. You can’t buy live barrel sponges or Acropora palmata, but you can get replicas and can then go on to stock the full range of Caribbean fish which would usually dine on such live corals.



The problem with marine biotopes is that it’s not always easy to ensure everything is from the same place. Live rock tends to be from the pacific, like Fiji, so no use for an authentic Caribbean biotope, and what about the corals? How can the OCD biotope creator be sure that what they are creating is all actually from the same place? The answer is to leave out the inverts. 

When creating a marine biotope I build it around the fish, and then once I have the fish I can build the aquascape and decor around their needs. 

A tip is that you don’t need to fork out on an expensive fish book either, and you wouldn’t always be guaranteed to find the information you need there as information on marine biotope aquariums is so scarce anyway. Instead just go on to google images and type in “fish of the Caribbean, or reef fish of Sri Lanka, or Red Sea reef fish for example. Within a nano second you’ll get millions of matches of old posters generated for tourists diving in those areas, and you can’t go wrong. Here’s when the fun starts…

A glance at a poster will now arm you with the kind of intel that a seasoned catfish keeper usually enjoys, the ability to decipher from a shop of mixed colourful fish where exactly each one comes from, and importantly, who usually lives next to who in their natural habitat. Setting up a tank with one of each of a real mish-mash of marine fish doesn’t really do it for me as it doesn’t offer anything more. No interaction, no natural behaviour, but challenge me to set up a Red Sea biotope or a Belizean one and my ears start to prick-up immediately.

What makes fish only biotopes even more interesting is that you can include all those invert munchers who would normally make the reef aquarist’s black list of definite no-nos for the home aquarium. Triggers, puffers, snappers, but also oddities like hogfish, squirrelfish and drums (tank size permitting). On a much less toothy level, your world has just opened up to some of the most iconic and beautiful of all the reef fish – large angelfish and butterflies. Now you can have that Emperor angelfish or Queen. Now you can buy up all those appealing butterflies and make sure they get a good feed too.

Photo by Pixabay on


Filtration for the fish-only aquarium can be as simple and inexpensive or as complicated and wallet busting as you want it to be. With no corals you are back to good old ammonia and nitrite conversion, which can be done with an external power filter or even an undergravel filter and powerhead if you want to. Trickle filters are still in for fish-only, as are wet and dry, and my only advice with all the filtration methods here would be to use some sort of surface skimming to remove that oily film which will build up. After that, its regular water changes to keep the end product of nitrification – nitrate – at low levels, ideally below 40ppm.

But why not try a slice of higher tech with a bio pellet reactor? These things were born to consume nitrate and phosphate and would positively thrive when connected to a fish-only tank. Put a modern protein skimmer inline first to remove dirt before it gets converted and you’ll run an even more pollutant free system. 

Many seasoned reefkeepers will have noticed that “natural” marine filtration methods often advise against the use of UV, as it will zap all that useful phytoplankton and bacteria upon which some corals feed, but not with the fish-only, and I advise UV on every fish-only tank, and lots of it. Ozone too if you want to, for even better water clarity and pathogen reduction. 

Can you add a refugium to a fish only? Absolutely, that algae growth will further counteract nitrate and phosphate and subsequent nuisance algae, and depending on the fish you keep above it, you could even feed the excess macroalgae to them. 

You know how catfish keepers sometimes get wood or rocks and put them somewhere to go green with algae before adding them to the main tank to be grazed? You could do this with a fish only refugium. Put good quality live rock in the sump, where it will grow beneficial algae, sponges and even critters if you don’t run copper medications, and then every now and then you can move the well-matured rock to the main tank for your angels and butterflies to pick at. 



The fish-only tank is definitely the place to get experimental with lighting. Lighting will set the mood in the tank and best of all, you won’t need much of it so low set-up and running costs. Choose LED lighting to offer those deep blue spectrums and spotlighting. For a dramatic aquascape light parts of the tank, not all of it, and that will also have the benefit of less light spread onto the glass, so less algae wiping. 30 watts of LED over a 4’x2’x2′ tank will be plenty, and it will look really cool too. You’d be looking at at least six times that much for a full blown reef so bring on the energy saving.



So get a massive tank and you might want to add all those large predators that you could never have with a reef, like puffers, triggers, moray eels and lionfish, but there are other options you may also want to consider.

How about a community of Butterflyfish, or again a massive tank needed, but what about the holy grail – a community of large angelfish?

Seahorses could be considered for fish-only, as long as they are the only fish that is, or what about something really weird like anglerfish and leaf fish?

Those damselfish won’t be quite so tyrannical when faced with a 10” angelfish, so they’re back on the list, and with no corals to worry about why not choose 50 green chromis instead of five, for a far more natural look?

A pair of tank bred clownfish in a fish-only tank is actually the most suitable and easiest marine aquarium of all, and highly suitable for beginners. What about a shoal of tank bred clowns like the public aquariums do for maximum effect? Remember most conventional rules go out of the window now and just like with Malawi cichlids, by crowding your angelfish, clownfish or damselfish you manage their aggression and prevent one individual from taking over the whole tank.

In the wild many large angelfish swim in pairs, as do some butterflyfish, and you’ll be able to keep pairs in your fish only. For the first time ever reefkeepers will be able to keep fish in multiples, and in densities approaching that of the wild.



One thing the fish-only tank isn’t though is a green light to keep absolutely any marine fish you lay eyes on. Just like with tropical fish, anything that grows to over 12 inches will be hard to rehouse and may have no value. Research size thoroughly and you may have some surprises. Still steer clear of sweetlips, pilotfish, most groupers and some wrasses. Fish must also be split and graded by you into aggression levels.

Some marines are very aggressive feeders – so aggressive that they should be fed with long reach feeding tongs and your hands should be nowhere their food when it is in the water. Again it’s the usual suspects – puffers, triggers, large wrasses and moral eels. All are very toothy, some could remove a finger and they will all lunge at and tear food with ferocity. This means that delicate butterflyfish won’t get a look-in at feeding time, so the two shouldn’t be mixed. I recommend any book by Scott Michael to help you here, as he will list aquarium suitability for virtually any fish you are likely to see, how difficult it is to keep, and even if it should be kept at all.



Busy fish-only tanks are the best tanks to incite a feeding response however, and many a seemingly impossible Moorish Idol and Cleaner Wrasse seem to thrive, not just survive, and I think it’s because of the natural competition for food.

Put a single wild-caught marine fish into a tank on its own, offer it some food, and often, it will refuse it. It’s in an alien environment, it has no other fish around it looking settled and importantly, it can’t see other fish feeding.

Put a wild-caught fish into a tank containing twenty or thirty other fish however and things become more like they are on the reef in terms of feeding. Food enters the space and the first ones to spot it and get to it, eat it. In the busy fish-only tank new fish not only quickly recognise that it is safe to be out and about and busily looking for food but also that when food hits the surface they need to race to get it, no matter what it is, or someone else will. Its the busy fish-only aquarium where supposedly tricky feeders start to eat dry foods, as they’d rather eat it, whatever it is, than spit it out and let their close proximity neighbours eat it instead.

And what a menu the modern marine fish has to choose from! Frozen foods are more varied, better transported and more nutritious than ever before but dry foods have taken quantum leaps. I remember a time when I wouldn’t dream of offering flake and pellet foods to marine fish but now I’ll go months at a time without offering anything else. Make no mistake – a good dry food diet can be far superior to a frozen food, nutritionally, and the way that the food is offered, like on grazer rings or pastes which you can push into rocks really is excellent, and offers the all important environmental enrichment to captive fish too.

close up photo of angelfish
Photo by Artem Mizyuk on

Holy grails of the fish-only world. Fish for lottery winners (the cool school)

Wrought iron butterflyfish, Chaetodon daedalma

Peppermint angel, Paracentropyge boylei

King angel, Apolemichthys kingi

Hybrid large angelfish

Hybrid butterflyfish

Angelfish with aberrant colour patterns

Clarion angelfish, Holacanthus clarionensis

Masked angelfish, Genicanthus personatus (also reef safe)

Conspicuous angelfish, Chaetodontoplus conspicillatus

Venusta angelfish, Paracentropyge venusta

Multibarred angelfish, Paracentropyge multifasciata

Narcosis angelfish, Centropyge narcosis (deep water species)

Cocos pygmy angelfish, Centropyge joculator 

Debelius’ angelfish, Centropyge debelius

Japanese pygmy angelfish, Centropyge interruptus 

Tinker’s butterflyfish, Chaetodon tinkeri

Yellow crowned butterflyfish, Chaetodon flavocoronatus

Marquesan butterflyfish, Chaetodon declivis

Screenshot 2019-10-13 at 12.20.06

Modern fish-only weaponry

Copper based medication

UV steriliser

Biopellet reactor

Superior dry food diet

Replica live rock

Fake corals

Recommended fish-only Butterflyfish and Angelfish species

Bannerfish, Heniochus spp.

Pyramid butterflyfish, Hemitaurichthys polylepis

Double saddled butterflyfish, Chaetodon ulietensis

Mertens butterflyfish, Chaetodon mertensii

Racoon butterflyfish, Chaetodon lunula

Klein’s butterflyfish, Chaetodon kleinii

white and yellow fish close up photography
Photo by Skitterphoto on

Threadfin butterflyfish, Chaetodon auriga

Dot-dash butterflyfish, Chaetodon pelwensis

Silver mono, Monodactylus argenteus

Queen angelfish, Holacanthus ciliaris (size warning)

French angelfish, Pomacanthus paru (size warning)

Emperor angelfish, Pomacanthus imperator (size warning)

Blue ring angelfish, Pomacanthus annularis (size warning)

Yellowbar angelfish, Pomacanthus maculosus (size warning)

Koran angelfish, Pomacanthus semicirculatus (size warning)

Sixbanded angelfish, Pomacanthus sexstriatus (size warning)

Cortez angelfish, Pomacanthus zonipectus (size warning)

Arabian angelfish, Pomacanthus asfur (size warning)

Although hardy, the above angelfish will need a minimum six foot tank when adult, ideally eight foot plus.


Advantages of the fish-only

Lower set up costs

More fish species choice in the shops

Copper based parasite medications can be used

Fish can be stocked in denser numbers

Conventional filtration methods can be used

Fish can be fed more heavily, and more frequently

Biotopes can be created more easily

High powered lighting need not be used

Air stones can be used

Lids can be used (less evaporation and no jumpers)

Corallivores (coral eating fish) can be stocked

Salt levels can be slightly lower (save money on water changes and helps prevent parasites)

Far better fish diets available than in the past (better fish colour, fish health and survival rate)



Jeremy Gay

Author of three fishkeeping books and lifelong fishkeeper. Experience includes editor of Practical Fishkeeping magazine, editor of Pet Product Marketing magazine, multi award- winning livestock manager and aquatic store manager.