Celestial pearl danios and emerald dwarf rasboras are two of the most attractive nano fish in the hobby, but which is right for your aquarium? We compare their care requirements, behaviour, and appearance to help you choose your favourite.
In the world of planted aquariums and nano fishkeeping, few species have captured the hearts of hobbyists quite like Danio margaritatus and Danio erythromicron. Native to Myanmar, these two micro-danios bring an astonishing amount of beauty, personality, and vivid colour to tiny aquatic landscapes despite barely reaching 2.5cm (1") in length.
Whether it's the intricate spotted pattern of the celestial pearl danio or the striking tiger-stripe pattern of the emerald dwarf rasbora, both species offer a captivating option for planted nano aquariums with harder water.
The question is, which species should you pick? Is it down to just personal preference of their patterns, or do their care requirements make one species more suitable than the other? Let's find out.
All photos by Shutterstock unless otherwise stated
Celestial Pearl Danio Overview
Danio margaritatus, commonly called CPDs for short in the hobby, were first discovered in September 2006, and it did not take long for them to become incredibly sought after in the hobby. In fact, demand was so high that the collection of wild fish was banned in February 2007 while experts assessed the population, as the numbers being found became increasingly low. You can read a more detailed summary of their emergence onto the fishkeeping scene here.
With their small size, dark blue base colour with an orange-gold spotted pattern, and vibrant reds on their fins, it is easy to see why everyone wanted to keep them. Their other common name is the galaxy rasbora, referencing how their flanks look like a collection of stars glowing against the dark backdrop of space.
They live to the east of Lake Inle in Myanmar and are found in shallow, clear ponds with aquatic plants and flooded terrestrial grasses. This natural habitat means they are well-suited to a densely planted aquascape.
Emerald Dwarf Rasbora Overview
Scientists have known about Danio erythromicron for much longer, with the species first being described in 1918 from Lake Inle itself. Reports suggest they became available sporadically to hobbyists in the 1970s before larger numbers started appearing in the 2000s (around the time CPDs arrived on the scene).
Lake Inle as an aquatic habitat is threatened by human activity. Deforestation is causing increased sediment runoff into the water, chemicals and sewage are discharged into the lake, and several invasive species have been introduced. So, it is no surprise that this species is considered endangered given it is endemic to the lake.
The emerald dwarf rasbora has blue and orange-pink stripes down the flanks, hints of orange at the base of their fins, and a dark spot at the caudal peduncle. Lake Inle is also home to the Asian rummynose (Sawbwa resplendens) and the two would make excellent tank mates. Aquatic plants are also abundant in the lake, with several species being introduced from elsewhere in the world along with plenty of natives.
Photo by George Farmer
Many years ago, George Farmer created a tank based on Lake Inle for Asian rummynose and the Inle Loach (Petruichthys brevis).
Confusing Names
Danio erythromicron was originally thought to belong to the genus Microrasbora. When the celestial pearl danios were first discovered, scientists believed they were close relatives of D. erythromicron. Hence why the names emerald dwarf rasbora and galaxy rasbora have stuck in the hobby despite now belonging to the genus Danio. In fact, for a brief time, researchers even placed them both in the genus Celestichthys due to how closely related they are.
Whatever name you see them listed under in your local fish shop, you can be confident you are looking at the right fish. Thankfully, their striking and distinct patterns make a mislabelled tank easy to spot from a mile away – well, maybe not a mile given how small they are, but you get the idea.
Water Chemistry Requirements
The obvious place to start when choosing one of two similar fishes is water chemistry, as it is always best to pick the one best suited to your water.
However, in this instance there is not a huge amount of difference between them given how close their natural distributions are. Both require water on the alkaline side of things, a pH between 7 and 8 working well, and moderately hard, up to around 20dH.
It is the same story when you look at temperature as well. Both like it on the cooler side of tropical, with 25°C (77°F) being their upper limit for long-term care.
Aquarium Setup
Their requirements in an aquarium are near-identical too. Both come from clear habitats filled with live plants, and so this is generally the best way to keep them.
The exact choice of plants is not critical, unless you want to create a biotope. Instead it's more about providing plenty of cover. They appreciate a thick, jungle-like aquascape where they can quickly, and easily, retreat into cover if they are scared.
Hornwort is a great option as its fine leaves are perfect as a spawning medium and it is found in Myanmar.
Hardscape choice is really up to the individual fishkeeper. Wood always works well and looks natural, and this is one instance that rocks that may slightly raise the pH and hardness of the water (for example Seiryu stone) can be used successfully without fear of the water becoming too hard for the inhabitants.
When it comes to substrate, again it is up to you. A planted substrate will work well, though, as it will provide plenty of nutrients for healthy plant growth and the dark substrate will enhance the fish's natural colours.
Related article: Simple biotope aquarium ideas for beginners.
Danio Tank Size
We've discussed the decorations, but what about tank size? Well, given these fish generally don't get larger than 2.5cm (1") a standard 45cm (18") tank (holding roughly 40 litres/8.8 gal) will suffice for a good size group in a species only display. If you want to mix them with other fish, you'll need to increase the tank size depending on which species, and how many, you plan to keep. A 60cm (24") aquarium (60 litres/13.2 gal) would work for shoals of both D. erythromicron and S. resplendens.
Behaviour and Compatibility
Again, both species are similar here too. They are small, social, and shy, meaning they'll appreciate the company of their own kind and can be easily intimidated by larger, or more boisterous species.
Larger groups will typically create the best displays with confident fish that will be easier to watch. Males can often be seen taking up small temporary territories to attract a female, and their displays are always eye-catching, whichever species you pick.
Inle loach - Photo by Tai Strietman
Keep them with other small, peaceful fish like rasboras, small loaches (the aforementioned Inle loach or the rosy loach being good options) and other similar species, or opt for a single species setup.
Possible Breeding Project
Dedicating a tank to these small danios not only allows you to keep larger groups, but it also increases the likelihood of one day spotting some fry appearing. Both of these species are known to breed in aquaria, and plenty of fishkeepers report fry appearing without intervention when kept in mature, planted aquariums.
For higher fry survival, a dedicated breeding tank is best. Fill a small tank with water from their aquarium and add fine-leaved plants like mosses, or some woollen spawning mops. A small sponge filter can be added, but it is not essential if you keep a close eye on the water quality.
Both species are continuous spawners, laying several eggs each day, which hatch after roughly three days. The adults will eat some eggs, so you can either swap out the moss/mops daily after spawning takes place, or remove the parents after a few days and hope that some of the eggs escaped their hungry mouths.
The fry should be offered infusoria or similarly tiny foods initially, progressing onto larger items as they grow.
So Which Species Should You Pick?
Ultimately, the choice comes down to personal preference. Their aquascape requirements, water chemistry, and compatibility are so similar that neither species has a significant advantage over the other. If you prefer bold, star-like spots, the celestial pearl danio is hard to beat. If elegant stripes are more your style, the emerald dwarf rasbora is every bit as rewarding.
Despite their similar requirements, I would resist the temptation to keep them together. Given how closely related they are, there is a possibility that they could hybridise, producing offspring that lack the distinctive characteristics that make each species so appealing.
So, whether you end up on Team Spots or Team Stripes, either species will make a stunning centrepiece for a peaceful planted nano aquascape.