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Additive: nutritive/non-nutritive ingredients that are supplemented in small amounts (alone or in combination) for a specific purpose, such as  to improve the quality of fish as a final product,  to preserve the physical and chemical quality of the diet, or to maintain the quality of the aquatic environment.

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1.  Antioxidant

2. Antimicrobial agents

3. Organic acids

4. Probiotics and prebiotics

5. Phytogenics

6. Color / pigmentation agents

7. Feeding stimulants and palatability enhancers

8. Color/pigmentation agents

9. Antiparasitic

10. Immunostimulating agents

11. Enzymes and hormones

12. Yeast

13. Mycotoxin binders

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A variety of additives have always played a role, either alone or as a combination, in compounded aquafeed.

Future research and trends:

1. Alternative of antibiotics. Because of the unpopularity of dietary antibiotics, the search for its alternatives like dietary immunostimulants, pre and probiotics, organic acids etc has become a research area of top priority nowadays.

2. Optimization of the additives.  Identification and dose optimization (species and age) of various additives are very important to enrich the practical use of these additives, from both an environmental and an economic perspective.

3. Feed to biomass. Feed transformation into biomass gain is a process that starts in the digestive system of the animal. As such, its health status and its functionality correlate directly with the economic results of the farmer.

Several studies have shown that different feed ingredients and changes in diet composition can affect gut structure and microbiota balance influencing digestive and absorptive functions (Ringo and Olsen, 1999) which in turns affect the health.

In our opinion, supplemented diet in aquaculture is important to reduce the grow-out loss in fish farming especially due to infections, also to enhance appearance, increase growth rate and maintain good health of aquaculture species.

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Definition: Carotenoids are the most importantsubstances responsible for color in aquatic animals.

Role: Physical appearance of cultured animals is a vital criteria affecting consumer acceptance and the market price.

Example: The pigmentation of goldfish and koi is improved by the addition of carotenoids; these fish are found to be capable of metabolizing zeaxanthin to astaxanthin.

Limitation: Synthetic carotenoids are expensive.

Improvement on future studies: Dietary lipid has been reported to have profound effects on pigmentation.

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Definition: Organic compound with acidic properties.

Role:  promising feed additive to improve gut health and performance.  The mode of action of organic acids in the intestinal tract involves two different actions: the pH-decreasing action of organic acids in stomach and small intestine contributes to an improved activity of digestive enzymes creates an impaired environment for pathogens and inhibits the growth of Gram-negative bacteria through the dissociation of the acids and production of anions in the bacterial cells

Example:  acetic, butyric, citric, formic, lactic, propionic, malic, and sorbic acids and their salts have been used as acidifiers in animal feeds

Limitation:  limited number of published studies on the use of acidifiers in growth promotion and disease prevention in aquaculture

Improvement:  results from the studies indicate promising potential and compel aquafeed manufacturers to consider using acidifiers. So, further studies about organic acids as feed additive should be done.

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Definition:  Phytogenic feed additives (PFAs) are plant-derived products which are added to the feed in order to improve animal performance.

Role:  stimulation of appetite, antimicrobial action, direct reduction of gut bacteria, stimulation of gastric juices, enhancement of immune system, and anti-inflammatory and antioxidant properties

Example:  Feed additives containing essential oils of aromatic plants are the most common type of phytogenic products used in aquatic feeds.

Limitation:  their efficacy is related to the type of plant, method of extraction, and extract concentrations

Improvement:  quantify and characterize chemically the plant extracts in order to identify active molecules, and facilitate the establishment of a standardized protocol, including extract concentration and adequate dosing.

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Definition: Live microorganisms added to the diet contributes for growth and immune response of aquatic organisms.

Role:

- Improve water quality and the health of aquatic organisms.
- Improve digestion by stimulating production of digestive enzymes (carbohydrases, phosphatases,
esterases) in the gut.

Example: Production of some of these enzymes has been enhanced by the inclusion of Bacillus subtilis in tilapia diets,  who further attributed
improved weight gain and feed efficiency to the increased enzyme production.

Limitation: To provide effective feeding and treatment regimens for all fish species.

Improvements:

- The viability of probionts during the feed-making process and during feed storage can be improved by encapsulation in nonnutritive matrices, such as calcium alginate.
- Further research on the dose dependency and form and the route of probiont administration on disease resistance is needed for all fish species to provide effective feeding and treatment regimens.

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Definition: A non-digestible dietary ingredient that beneficially affects the host by selectively stimulating the growth of and/or activating the metabolism of
health-promoting bacteria in the gastrointestinal tract.

Role:
- Increase in beneficial bacteria and  decrease in harmful bacteria in the gut.
- Modify the microbial community within the gastrointestinal tract to boost nonspecific immune responses.
- Improve growth performance and feed utilization of various fish species.

Example: The growth rate, feed efficiency, and survival were improved in two experiments involving rainbow trout (Oncorhynchus mykiss) that were fed a diet containing 2 g/kg mannan oligosaccharide compared with those fed the basal diet.

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Definition: Compounds that can control, prevent, or delay microbial growth in cultured animals, as well as in feed itself.

Role: Medicated feeds have been accepted as more effective and safer methods when compared with injection and bath treatments to control or prevent disease in aquaculture.

Example: common antibiotics, such as oxytetracyclin (OTC) can give growth-promoting effects including improved nutrient absorption due to thinning of the gastrointestinal lining and decreased numbers of harmful bacteria in the intestine.

Limitation: antibiotic use in aquafeeds is becoming unpopular as research is increasingly shedding light on the associated dangers and drawbacks, such as the emergence of antibiotic resistant bacteria as well as threats to human health due to antibiotic residue in the animals

Improvement: Extensive research has been carried out to find safe and natural dietary additives for eco-friendly aquaculture like natural antimicrobial products. However, the efficacy of these natural antimicrobial compounds in fish feed remains an important research area that needs to be investigated.