Formulated (Dry) Diets

If you are unable to culture or feed live foods to your larval rainbowfishes, you can still get reasonable results rearing them entirely on a fine powder-based diet manufactured by a number of commercial suppliers. Advantages include ‘off-the-shelf’ availability. However, artificial diets are ingested at a lower rate than life foods and are negatively buoyant in water. Generally, rainbowfish larvae fed only dry formulated diets often show lower survival and poorer growth compared to those fed live foods and can often lead to higher incidence of deformities. Non-living feeds do not yet have an advantage over live food organisms.

During recent years intensive research has been conducted by a number of commercial suppliers around the world to develop micro diets that can partially or fully replace the use of live food. Substantial advances have been developed and commercial micro-diet formulations are becoming more and more successful as a partial replacement for live foods.

Two types of micro-particulate particles have been used for manufacturing formulated diets for feeding fish larvae; these are (1) micro-encapsulated diets and (2) micro-bound diets. Both have been used extensively in nutritional studies with fish larvae. The major difference between the two is that micro-encapsulated diets have a membrane or capsule wall which separates dietary materials from the surrounding medium. The capsule wall helps maintain integrity of the food particle until eaten and helps maintain water quality; however, it may restrict leaching of water soluble dietary components and therefore reduce the attractability of the food particles. The capsule wall is also thought to impair digestion of the food particle and a number of studies have reported poor growth and survival of fish larvae fed microencapsulated diets.

Micro-bound diets consist of dietary components held within a gelled matrix or binder. They do not have a capsule and this has been suggested to facilitate greater digestibility and increased attractability through greater nutrient leaching. Many different binders have been used in micro-bound diets including polysaccharides from seaweed such as agar, carrageenan and alginate and proteins such as zein and gelatine. They vary considerably in their properties and nutritional value and choice of binder can significantly influence the rate of ingestion of formulated food particles and nutrient assimilation. Water stability of micro-bound diets is also influenced by the binder employed. Microbound diets made from agar and alginate was amongst the most stable in terms of integrity, while carrageenan was amongst the poorest.

Prepared diets should be of a very fine grain, nutritionally complete, palatable, be less than 100 microns (µm) in size, and remain floating or suspended in the water column. I have found OSI® Micro-Food an excellent first food for rainbowfish larvae. OSI Micro-Food is a microencapsulated (50−150 µm) diet rich in protein, fatty acids, and vitamins. OSI Micro-Food or artificial plankton (APR), as it is known in the aquaculture industry, was designed for feeding prawn larvae, but it works well on larval rainbowfishes too. Just use it dry and sprinkle it over the surface of the water. Other well-known products such as Sera Micron® and TetraMin® baby fish food are also suitable first food for rainbowfish larvae.

It is important that the feed size increases as the larvae grow until they are large enough to take regular flake and pellet feeds. It can be advantageous to mix the sizes for a few weeks, especially if there is size variation within the larvae. Care should be taken not to overfeed, as prepared diets tend to easily decompose and pollute the water if overfed. Just a small tip here, keep all your fish foods in the refrigerator, they last longer.

Another simple, less expensive but very efficient method to feed larval rainbowfish is hard-boiled egg yolk. Just wrap the yolk in a clean cloth (or old nylon stocking) and twist it down into a section of the cloth. Then all you have to do is swish it around in the water. The egg yolk will keep in a refrigerator for a few days. An even easier option is to purchase some powdered egg yolk from your local baker.

Powdered yolk can be stored in the refrigerator for several months. When you want to feed the yolk just sprinkle it dry on the aquarium water surface. However, low survival rates of rainbowfish larvae fed egg yolk alone have been observed, which suggests that its use in larval rearing needs to be re-evaluated. Rainbowfish larvae which were fed egg yolk with brine shrimp nauplii and microworms recorded much higher survival rates.

The copepod, Diaptomus connexus (sold freeze-dried under the commercial name of Cyclop-eeze®) is considered to be another excellent food for juvenile rainbowfishes by many aquarists. Cyclop-eeze is a commercially available product marketed by Argent Chemical Laboratories. Its actual purpose is the replacement of brine shrimp, spirulina and artificial plankton (APR) as starter diets for larval fish and shrimp.

The manufacturer describes Cyclop-eeze as a superior natural product, suitable for almost any aquaculture purpose. The copepods the product consists of “are selectively bred, biologically engineered micro organisms which are cultured in a pristine arctic lake”. The decapods harvested for the production of the product are described to be of “blood orange” coloration. The company explains this with the high level of astaxanthin. The importance of carotenoids in fish diets has been stressed by several researchers working in the field of fish nutrition. In addition to the high content of carotenoids, high levels of highly unsaturated fatty acids (HUFA) are also found in the copepods. All polyunsaturated fats are increasingly recognised as important to animal health. High levels of omega-3 HUFA have been described as favourable for larval aquaculture applications and typically are obtained by food supplements.

Cyclop-eeze is also seen an interesting product because it occurs to be rich in the attractant betaine as well as other palatability factors. Betaine as well as carotenoids and phospholipids are widely used as attractants and stimulants. Unfortunately, not a lot of research on Cyclop-eeze has been published. The information available on nutritional values and compositional aspects of Cyclop-eeze are therefore limited to the manufacturer’s information and little independent research has been conducted evaluating the product in fish feeding.

However, there are now numerous commercial products available for feeding fish larvae and they are all certainly worth trying. Recent developments have included the use of a newly formulated larval micro-diet (Gemma Micro®) that has helped reduce brine shrimp usage by more than 95 per cent. This diet has proven to be more attractive and beneficial to the fish.

You need to try these products yourself and see if they are suitable for your situation. Evaluating and combining different food products can provide suitable diets for any of the fishes we maintain in captivity and can closely resemble the wild diets with which they have evolved. This should minimise nutritionally related health issues and improve your success as well as the general wellbeing of the fishes.