By Mach Fukada, HAS (reprinted from May 1998 I'A O HAWAI'I)
Pseudomugil cyanodorsalis, is probably one of, if not the most beautiful blue eye that the average person will ever see (I consider myself the average person and I have seen it and have been able to breed it). It is about the same size or a bit larger than a neon tetra (it is one of the smaller blue eyes), but by far a more intense blue (if you can imagine that). The remaining body is a bold contrasting brassy orange/yellow color, which is further offset by black accents on most of the fins. P. cyanodorsalis originates from the northern part of Australia and it has been conjectured that it will also be found in southern Papua New Guinea. This fish is found near the mouths of streams, in waters that range in salinity that vary between fresh to 100% seawater.
I was often told that it was one of the most rewarding and beautiful fish to raise by several Honolulu Aquarium Society members and this stirred my curiosity (I figured I was already over the limit of 6 species of rainbowfish set by my wife, so why not?). Thus began my attempt at keeping this jewel from Australia.
Prior to making an acquisition, I had been checking various resources for information on this fish. These sources included Dr. G. Allen's book "Rainbowfishes in Nature and in the Aquarium" and various sites on the internet: Rainbowfish Online , Rainbowfish Study Group, Aquarium Mailinglists (The internet makes finding information very fast and trading/selling fish easy. Many people are looking for fish or plants we have here in Honolulu and are very willing to do trades and many will give fish away for the cost of shipping via express or priority mail).
All of the sources indicated that this fish preferred a higher salt concentration than most, but could be adapted to fresh water over time. I was already playing with another blue eye that preferred 50% seawater and figured "why not". So a friend of mine (I met via the internet) sent me a bunch of eggs with instructions regarding hatching. He suggested hatching them in 75% sea water. So I mixed up some "Instant Ocean" and diluted down to 3/4 strength in a 10 gallon tank with a sponge filter. Many of the resources indicated that a the addition of crushed coral would add calcium and help keep the water well buffered. My various contacts indicated that although it is possible to use 100% rock/sea salt to mix up a sea water solution, it is not recommended. The comercial mixes add the necessary amounts of trace elements (Magnesium, Calcium, etc.), which may be absent in coarse/rock or "hawaiian" salt. Apparently, most blue eyes and rainbows are fairly susceptible to deficiencies in Magnesium or Calcium.
At about 2 weeks nothing had hatched and I checked with my friend and he suggested to check if the eggs were developing and "force" them if they were ready. A careful inspection revealed that the embryos in the eggs were well developed, I could see their eyes looking back at me as they moved around in the egg shells. I removed them from the tank and placed them into a small vial (film can) with some of the tank's water. I blew some air (I prayed that halitosis wouldn't kill the eggs) into the vial, resealed it, and placed it into my pocket, and walked around pausing once in a while to shake the vial. The theory behind forcing is that an increase in CO2 concentration and agitation will cause the fry to secrete an enzyme that breaks down the egg shells, thereby releasing the fry. (This procedure is commonly used with killifish.) IT WORKED!!! I had about 12-15 little fry swimming around looking for something to eat. The fry were bigger than most rainbows and were able to eat baby brine shrimp (BBS) right off the bat. I supplemented with a little Artificial Plankton Rotifer (APR) just in case there were a few smaller ones that could not eat BBS. The odd thing was that BBS did not die in 75 % seawater, they would thrive and grow. Thus, over feeding can be a problem. The fry reached adulthood in about three months after hatching. The adults ate flake food in the morning and BBS in the evening. At this point, I gradually reduced the concentration of seawater. Now I have them in 25-50% seawater. I have been told that they can become acclimatized to 100% freshwater, but this may have negative side affects like reduction in life span and reproductive potential.
My adults (7 males and 6 females) would produce about 30 eggs per day. They spawned like a typical rainbow or killie on yarn mops. They seem to prefer mops that are light brown or beige. What is troublesome is that they will often completely ignore the mops and lay their eggs in the gravel (I make a point of straining the water I siphon out of the tank to collect any eggs in the gravel). The eggs hatch in about 2 weeks and fry are very easy to raise.
I hope I haven't scared any one away from trying to keep this marvelous small blue eye. It is easy to care for save its preference for brackish water. It is well worth any of the minor inconveniences of dealing with brackish water.