Treating adult fish: therapeutic protocols
Beyond the protection of eggs, methylene blue is a genuine first-aid kit for the adult fish. It acts on three fronts: as an antiparasitic (against external protozoa), as an antifungal (against moulds) and, in a lesser-known but vital role, as an antidote in cases of asphyxia (nitrite poisoning).
1. Nitrite poisoning (methaemoglobinaemia in fish)
This is the most urgent and most spectacular use of methylene blue.
When a biological filter fails (a spike of nitrites NO₂), the nitrites pass into the fish's blood and oxidise the haemoglobin, preventing it from binding oxygen. The fish literally suffocates, even if the water is saturated with oxygen (the gills are often brown instead of red).
Emergency treatment:
Methylene blue restores the blood's capacity to carry oxygen.
- Action: immediate.
- Protocol: isolate the fish in a hospital tank with a standard dose (1 to 2 mg/L). An improvement in breathing is often visible within a few minutes. This is the same biochemical mechanism as in humans (see human health).
2. External parasites (protozoa)
Methylene blue is effective against several single-celled skin parasites, in particular:
- Ichthyophthirius multifiliis (white spot): effective, especially when combined with a rise in temperature, although malachite green is sometimes preferred for severe cases.
- Costia, Chilodonella, Trichodina: these microscopic parasites cause cloudiness of the skin (a greyish veil) and rubbing behaviour.
3. Fungal infections (fungi)
On an open wound or after rough handling, a cottony mould (Saprolegnia) may appear. Methylene blue halts the progression of the fungus outright and prevents secondary bacterial infection of the wound.
The two methods of administration
It is crucial to choose the right method so as not to stress the animal needlessly or destroy the balance of the aquarium.
A. The flash bath (high concentration, short duration)
This method is ideal for clearing parasites from a fish or treating a localised fungal infection without treating the whole tank.
- Container: a bucket or a basin filled with water from the aquarium.
- Shock dosage: a high concentration, up to 200 mg/L (that is, about 10 ml of a 2% solution per 1 litre of water). The water is midnight blue and opaque.
- Duration: very short! 10 seconds to 30 minutes maximum. Watch the fish at all times. If it rolls over or gulps for air, return it to its normal tank immediately.
- Advantage: a drastic action on external parasites, with no pollution of the main tank.
B. The long-duration bath (low concentration, in a hospital tank)
For nitrite poisoning or sustained treatments (white spot).
- Container: a hospital tank (a bare aquarium, with a heater and an air stone, without a carbon filter).
- Standard dosage: 1 to 2 mg/L (the water is light to medium blue). See preparing solutions.
- Duration: 3 to 5 days. Renew 50% of the water every 2 days, adding the proportional dose of blue.
- Advantage: a gentle treatment, with less osmotic stress.
Precautions and interactions
- Never in the main tank (if possible): the blue kills the filter bacteria and stains the seals (see filtration and common mistakes).
- Interactions: never mix it with other treatments (formalin, copper, antibiotics) without expert advice. The cocktail may be toxic.
- Light: the blue degrades in light. Switch off the lighting of the hospital tank to maintain the effectiveness of the product.