Anaesthetics: Types of anaesthetics


There are many agents used to anaesthetize fish. Obtaining supplies of anaesthetics will depend on location – but all veterinarians should be able to obtain MS222 for you. Some of these anaesthesia agents can be obtained from the local pharmacy – or mail order from hobbyist magazines.

MS222 (tricaine methane sulphonate) is the only product licensed for fish use. It is a white water-soluble powder which is stable when kept cool and dry.  It is a derivative of benzocaine (see below) A standard stock solution is made by adding 10g of tricaine to a litre of water – which should be stored in a dark container as it is unstable in sunlight. MS222 solutions are acidic and therefore the pH of the anaesthesia solution needs to be checked prior to use. In poorly buffered water it may be necessary to buffer the standard stock solution using sodium bicarbonate – maintaining the pH between 7 – 7.5.

MS222 is an hypoxic agent- so therefore the anaesthesia vessel should be vigorously aerated in use. Tricaine is used at a rate between 50 – 100mg / litre, which means adding 5-10 mls of standard stock solution to each litre of water used. Fish would normally recover in ten minutes or less.

Benzocaine (ethyl aminobenzoate). A colourless crystal or white powder that is a popular anaesthetic for fish. It is cheaper than MS222. Benzocaine is poorly water-soluble and  needs to be prepared in either ethanol or acetone.

It has a reasonably wide margin of safety and doesn’t require buffering. Benzocaine is more toxic at warmer temperatures. The stock solution should be kept in a dark container, and like tricaine it acts as a hypoxic agent, so aeration is required when it is used.

A standard stock solution can be prepared by adding 100g of benzocaine to one litre of acetone/ ethanol. This is then used at a rate of 1ml per litre of water.  At these levels the fish should lose equilibrium in in 2-4 minutes. Fish should recover in ten minutes or less.

Eugenol or clove oil is often available over the counter at many pharmacies. It is not completely water soluble and should be mixed with ethanol or acetone at a standard rate of 1:10 (i.e. 100ml eugenol to one litre of solvent) This gives a working solution of 100mg of eugenol per ml of stock solution.

It is used at a rate of 40 -100mg per litre which equals 0.4 – 1.0 ml of stock solution per litre of water.

Other anesthesia agents include: Quinaldine Sulphate – light yellow crystalline powder, more expensive than tricaine, water soluble and needs to be buffered as tricaine. Its advantages are rapid induction and recovery. Reported to a gill irritant. Although fish lose equilibrium, there is not a total loss of reflex response.

2-phenoxyethanol (phenyl cellosolve) used at a rate of 0.1 – 0.5 ml / litre. It has a narrow margin of safety. Although cheaper than many anaesthetics it has adverse side effects such as a long induction time and erratic swimming. It may cause liver and kidney damage

There are several other agents used, but these are not readily available. The other option available to hobbyists in some areas are proprietary preparations. For example in the UK, NT Labs now market Koi Calm