Breed 500 or more Cardinal Tetras

Photo by Phoenix44

With this article I hope to encourage FNZAS members to have a try at breeding one of the most popular tropical fishes worldwide – the cardinal tetra, Paracheirodon axelrodi. Cardinals have been spawned in NZ but, to my knowledge, this has not been maintained on a regular basis. From time to time since obtaining my first cardinals about 1960, successes have been obtained in my fish room but not continually such as with other tetras including neons, rummy-noses etc.

The cardinal tetra was introduced to aquarists in the late 1950’s and was considered to be the most strikingly coloured freshwater fish since the arrival of the neon tetra in the 1930’s. A large school of cardinals in a suitably aquascaped aquarium is a stunning sight, particularly if attention is given to subdued lighting, dark substrate and quality water.

Cardinals are frequently available in many NZ retail petshops, usually priced from $5 – $10, and are virtually all from wild caught stock. They are not a cheap fish compared to, say, the neon tetra at $1.50 – $2.50 each. This is probably why many NZ aquariums, although displaying a few cardinals, seldom house a large school of 50 – 100 which would give a much more visual impact. The main reason for this is cost, as the hopes of commercial aquaculture breeders overseas to mass produce cardinals following their introduction has not, to date, been achieved – unlike what has been done by the Chinese in rural Hong Kong with the many millions, per annum, of neon tetras bred for the world markets.

Following the discovery of cardinals by Dr Harold Sioli in Sept. 1952, near Sao Filippe in the Rio Negro region, wild caught cardinals supply the world markets. Many millions are exported from Brazil, and to a lesser degree Colombia, each year (40,000,000 in 1996). This is an important income earner for the local populace centred around Barcellos on the Rio Negro, 386kms upstream from Manaus, where most are flown overseas to world markets.

There is concern that a large number of cardinals captured do not survive the first stages of export (up to 50 million PA) and efforts are now being made to reduce such losses. Further more, due to the large demand for this species (which is the highest exported ornamental fish by far from Brazil) they are getting harder to catch. What took one day some years ago, now takes one week for the same number, and the fishermen must travel further afield. There are concerns that more attention must be made to control the fishery, which could mean a limit on volumes taken and closed collection periods, to protect cardinal exports in the future.

For the intending breeder of this fish, a study of the bio type is of interest. Cardinals do not habit the main Rio Negro river or its tributaries but rather their source in dense forests and swamplands. In the rainy season, December to March, when many waterways and lowlands become flooded, cardinals move further upstream over areas that were dry, or at best only swampland, in the drier months. This is the time when cardinals breed, stimulated by fresher water, extra food availability, and temperature changes.

A common myth is that cardinals breed in water that is dark brown and very peaty with low acid readings more relevant to the water found in main rivers in the dry season. The water, however, at all times in this area is very soft due to soils having the lowest mineral content in the world. The water during the heavy rainy season, when the cardinals breed, is therefore further softened and acidity diluted as shown in low hardness and conductivity readings. Another factor to be noted in this bio type is that cardinals are found only in dark and shaded waters away from sunlight and they have a distinct dislike of strong light.

If you are interested in producing, say, 500 cardinals, now is the time to purchase some breeding stock. About 20 will be needed and should be young imported fish, not old fish from established aquariums. Set up a 60 x 30 x 30cm aquarium with a black painted bottom, back and sides, subdued lighting, and soft water. Use no substrate which could harden the water. Plants can be kept held down with waterlogged driftwood. A plastic box type filter is appropriate using glass wool and a little peat to slightly acidify the water but provide water changes as needed to keep the conductivity low ie Below 200 micro-siemens.

A varied diet is needed including daphnia, white and blood worms, and particularly brine shrimps, to bring our growing cardinals into breeding condition. Consider setting up a sea monkey tank to provide larger brine shrimp adults for your breeding stock. I have a 45 x 23 x 23cm tank sitting on a window sill in the fish room containing salt water of .130 specific gravity and provided with strong aeration. The shrimps grow to about 1cm in a month if fed on a yeast and water solution which I keep in a plastic 1litre lemonade bottle. Feed a little regularly only when the salt water tank clears by the shrimp eating the food.

As we will be testing the hardness and pH values of our water conditions, I would suggest the purchase, or acquisition, of a conductivity and pH meter. I have one of each which I use for quick and accurate tests when required.

Raising the Fry

With a lot of tetras, obtaining the eggs is the easiest part. Raising large numbers of fry is the real challenge!

After 5 days, and not before, remove the covers and check for results; either with a torch, or place the tank on a shelf with light from behind. If all has gone well, we can see small fry shooting around the bottom edges of the tank and through the peat fibre. There will be many more hiding in the peat so don’t think that sighting only a small number is necessarily a poor result.

At day 6, when their yolk sacs are used up, they should be free swimming and must be fed. Use a magnifying glass to check if the fry are actually swimming, rather than just hopping about while still having some yolk sac left. Water temperature will have an effect on this time as a higher temperature will shorten the hatching process.

This is a critical stage when many, or all, of the fry can be lost due to starvation, feeding too late, too soon, or too much resulting in sudden water pollution.

Cardinal tetra fry can take brine shrimp from day 1 as long as it is the right sort. San Francisco Bay brine shrimp eggs are the ones to use as these are significantly smaller than the Great Salt Lake, Utah, eggs. SF Bay eggs are in demand by the aquaculture industry world wide, mainly for marine and shrimp aquaculture, ahead of the freshwater hobbyist. Don’t be fooled by packets sold as San Fransisco Bay BRAND and other labels without checking the source of supply. You will find that they ‘originate’ in Utah ie. The larger eggs. The shrimp to feed our cardinal fry on days 1 – 4 must be ‘just hatched’ not 12 hours or more after hatching as they will have grown to a large size and are too big for the little mouths to take. Use clean sea salt water and position jars in a sunny and warm place, providing strong aeration, to get a good brine shrimp hatch.

Feed fry morning and late afternoon, but only small amounts. Use an eyedropper measuring a drop or two into a glass of water to inspect the volume of shrimp before carefully adding to our fry tank. After half an hour, check that the fry have orange bellies; If so, we are over the hard bit unless we get slack and overfeed with brine shrimp which will pollute the water. If there are signs of orange shrimp on the peat, you are overfeeding. After 4 days use any brine shrimp, and microworms, if available.

After 4 or 5 days commence removing, and adding, small amounts of matured water at the same temperature and conductivity each few days to raise the tank level up to 20cm. I have a 15mm diameter plastic tube siphon which has a bulb shaped gauze end wrapped with a piece of nylon stocking for these jobs. The nylon is too fine for fry to be sucked up but inspect it for holes regularly.

After 2 weeks, and when there is sufficient water height, add a small sponge type filter which will help keep the water in top condition. From the 3rd or 4th week the fry should be swimming clear of the peat and start to show some colour and hopefully we have produced +100 young cardinals.

Subsequent spawning from our breeding stock should produce higher numbers. Don’t try to separate fry from the peat until they are over 10mm. Careful use of a larger plastic siphon, to remove peat and water from one end of the tank, is probably the best method. Check the contents after it has settled for any fry inadvertently picked up, and catch these with a small net.

When moving fry around to larger growing tanks, ensure the water quality and values are similar using your conductivity meter. Young fish do not take kindly to sudden changes and our hard earned labours can result in heavy losses. Always balance the water when moving from one tank to another.

Keep a record of details of your fish breeding attempts as they can be useful in the future. At the end of this article is a copy of the form I have used on occasion and may be of assistance. I wish every success to those of you who are going to give breeding cardinal tetras a go. I look forward to reading in the Aquarium World listings from the breeding co-ordinator of the beautiful cardinal tetras being recorded on many occasions. Who knows when the wild supply will be fished out or restricted by price (or whatever other reasons) to the NZ hobbyist.

Breeding Methods

When your cardinals have grown to adult size, 30 – 35mm, and about a year old, the fish will need to be sexed and placed in separate aquariums, or separate your growing aquarium with a glass partition. The females must be heavy with roe; hold them in a net up to the light and you will see eggs in the body cavity. Males have a thin, pointed cavity and a slimmer profile.

In the meantime, whilst you are feeding your split breeders, set up the breeding tanks. I use 45 x 23 x 23cm glass tanks which are sterilised with potassium permanganate (KMnO4) – including airstones, tubes, cover glass etc. I would suggest setting up three tanks, so that we can increase our chances of a hit, or maybe to six tanks if you are the determined type like me and want results.

For the substrate and egg catchment, I use peat moss that has been water logged for some months and then boiled up to achieve sterilisation. The cooled peat can then be spread into the breeding tanks to a depth of 2cm.

Previously collected and matured rain water, with a conductivity of 20–60 micro-siemens, is then added slowly through a small plastic tube, to avoid stirring up the peat, to give a depth of 12cm. Leave the tanks in a dark place with cover glass on, and very slight aeration, for 2 weeks. The water should now be clear with a slight brownish tinge but the conductivity should not have changed more than a degree or two. The peat will provide a natural substrate and hiding place for the eggs; act as a bacterial reducing agent, and will slightly lower the pH value of our rain water. A pH value of 6.5 – 6.6 is about right. I do not use nylon mops or plant.

Concerning temperature, for those of you who do not have a heated fishroom, we need to aim for a water temperature in the high 70’sF (26º – 27ºC). The problem with having a glass heating element in the spawning tanks is that they will burn small hatched fry if they hide and settle on the heater. Better to use heating pads, as discussed in a recent Aquarium World article.

To keep tanks in subdued light, I wrap them in black polythene with a window strip cut out in the front to about halfway down to observe for any eggs.

I introduce two females and two males to each tank hoping that at least one will drop her eggs, which is usually after the third day. Cardinals take some time to settle down to new surroundings and the peat substrate and masked sides of the tanks will help with this objective. Community spawns have been achieved but this can result in smaller numbers due to other fish using the occasion for an egg feast. Fertile eggs are clear and non-adhesive.

Cardinals spawn at night, so look for the odd white unfertilised egg or two on the peat each morning with the quick use of a torch and eye glass. Do not feed the fish in the spawning tank. If you see the odd white egg, your cardinals have spawned and many eggs will be hidden in the peat, especially round the edges of the tank. Carefully remove the adults and cover the tank completely to block out all light. After the spawning, I add 5 drops of 5% Methylene Blue to help sterilise the water from bacterial growth.

The female fishes which spawned should be a bit slimmer and are the ones to keep identified in a separate holding area. These fish, if well fed, will breed again in about 14 days. As with other tetras, it is essential to regularly spawn these young, breeding females to allow for continuing egg production and not to stagnate and harden the eggs in the ovaries of unmated females. Better numerical results of fry are achieved after one or two spawnings. I have bred cardinals without peat substrate using 3mm stainless steel mesh grills to protect falling eggs from hungry parents. You will need to hold rain water in a large plastic container with the peat bagged in a nylon stocking which is hung in the water for a month or two. When the water is clear, with a brownish tinge, the pH about 6.5 and conductivity 20–60 micro-siemens, set up your breeding tanks as before. I, however, prefer to use the peat method, despite separation problems with the young fry later, as the peat is a more natural substrate and tends to relax breeders more than in a bare tank.

Some points which I consider facilitate success; My best results have been during hot summer months when day temperatures in the fish room were over 26ºC. Full moon periods when spawnings resulted have been noted. Consider the use of a very low wattage light overnight, some distance away from your breeding tanks, to simulate moonlight. The use of slightly older male fish that may have bred before seem to be better drivers of younger, virgin females.

If no eggs are seen after 7 days, remove the adults and return them to the holding tanks for further conditioning. Try again after a few weeks. Try replacing most of the water with matured rain water and next time use different fish, ensuring the females are full of roe.
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