top of page

Boraras Urophthalmoides -- the “Exclamation Point Rasbora”

By Don Kinyon

A tiny jewel from Thailand

Kottelat 1991

It’s always a good rule to know a new fish’s needs before you decide to take it home. Sometimes it just doesn’t work that way, so thank goodness for the internet.

For some of the bigger events our aquarium society holds, we have a “live fish raffle” as one of the activities to cultivate interest in new and different fish. The way my luck works, my purchase of tickets is more like a donation to a good cause that it is a chance to win, but this time it was different: I won first choice! One of the species available was Boraras urophthalmoides, the exclamation point rasbora. Not being familiar with the species, I looked them over and was taken by the nickname, along with the brilliant metallic green line the fish displayed, even though they were confined to a plastic bag. Thanks to the web and to cell service, I knew enough about the fish by the time we left the meeting to be fairly confident I could keep the fish alive.

Boraras urophthalmoides is native to swampy areas near the Sai Buri River in Thailand and possibly south to Malaysia. The soft water is fairly to very acidic, depending on the season, and there is very little current. Temperatures vary from the low 70s to above 80 degrees.

This is a very small species, the largest getting to around three quarters of an inch in length. The body is slender, with healthy females being somewhat more robust, but still a slimly shaped fish. There is a black line running longitudinally along the body, ending just before the tail, and a black dot on the caudal peduncle, forming a horizontal exclamation point that gives the fish its nickname. The black stripe is flecked with metallic green and just above this line is a metallic stripe that can be anywhere from red to yellow to gold to green. When the fish are healthy and feeling at home, the stripe is brilliant. The fins are clear for the most part, though the first few rays on the dorsal and anal fins on some individuals are black.

The only space that was available for this group, I believe there were eight in all, was a standard 5.5 gallon tank on the top shelf of a rack. As it was, there was straight rain water in the tank and the temperature was around 75 degrees. A thin layer of fine sand covered the glass bottom, a small sponge filter was in one corner and two-thirds of the tank was jam-packed with greenery: a mix of any plants other tanks could spare.

Once acclimated to the temperature, the group seemed to be under no stress and appeared to be in search of their next meal. The water tested at 5.5 pH and 16ppm TDS: soft and acidic, hopefully near the quality to which the species is accustomed.

Conveniently, it was warming up outside and the daphnia were appearing in outdoor pools. Mosquitos were making egg rafts as well, so the new fish had a varied diet and grew heavy. I found that the larger of the daphnia along with the more mature mosquito larvae were too large for the little fish to eat, so the larger larvae and pupae had to be taken out and given to larger fish. It’s never good to allow a large number of mosquitos to hatch out in your fish room and spread though out the house! (speaking from experience) Removing the mosquito egg rafts from the outdoor pools and adding them to the rasbora’s small tank gave the fish a fresh supply of food for a day or so: as the eggs hatched and the mosquito larvae emerged, the small fish would snap them up in no time. Newly hatched brine shrimp were added to the fish’s diet, which they ate with just as much gusto.

All went smoothly with water changes once a week at around 50%. All the fish seemed healthy and were eating well. The females grew round with eggs, but I never noticed spawning activity or saw any fry. Thinking that the temperature may be too cool for the fish to be triggered into spawning, a small heater was added and temp brought to 78 degrees.

About a week later, a night time check an hour after the lights went out revealed a tiny fry in an upper corner of the tank. It looked like a tiny shard of glass attached to the aquarium glass. It wasn’t clear to me that it was a fry, but once the light had shown on it for a few seconds, the fish moved away to another spot on the glass. There was (at least one) fry in the tank!

At this point, green water from outside tanks was added every day, or rotifers from an inside culture. The young fish were rarely visible during the day, seeming to be shy and hiding in the dense foliage that by now was filling a good share of the tank space. Every once in a while a half-grown specimen or younger fry could be spotted, but most of the time it was only the adults that showed themselves during the daytime hours.

This went on for several months. It wasn’t clear just how many fish were in the tank until I removed all the greenery one day. The group of adults or near adults had grown to around thirty, and there were young fish from tiny fry to those nearing maturity, a rough guess would put the number at seventy to eighty total individuals in the five-and-a-half-gallon tank!

My original plan was to clean the tank out, give the fish away and start over with something new, but as successful and low-maintenance as the system turned out to be, I only ended up giving away a portion of these tiny rasboras and kept the rest.

At this writing, the small tank full of small fish remains as it has since the beginning and the fish still thrive. Every two or three months some of the younger adults are removed and given away to make room, and it is usually a surprise how many young have been produced in the meantime. I’m sure there are more efficient methods of raising the tiny jewel, Boraras urophthalmoides, but for a lazy fishkeeper, this system works wonderfully!

Recent Posts

See All

The Best Sunnie in the Show [Sunfishes]

by Bob Bock Many native fish are like spring flowers. Tulips, daffodils, and crocuses erupt from the earth in late March or early April, blazing with brilliant hues of red, yellow, white, and purple.


bottom of page